As a Dark Elf player, some comments: (+ means better for points cost with new rules; - means worse for points cost with new rules than before)
Just to clarify murderous prowess special rule specifically excludes mounts on the first page where the rule is discussed, so even if a model with mount has murderous prowess the rule only benefits the riders and not the mount.
Gains:
ASF (high I means re-rolling to hit against most opposing units, especially lizardmen; executioners no longer need an ASF death hag BSB in unit)
Murderous Prowess (re-rolling 1's to wound is under-rated., the weakness of S3 and S4 elves ia partially addressed increasing the rate of wounding by at least 1/6)
Dark Steeds are fast cav, meaning that characters on dark steeds can vanguard with fast cav units. and gain feigned flight rules (can march and shoot if they rally after fleeing a charge, instead of merely reforming)
Loss:
No longer have hatred except against high elves and Black Guard. Not much of a loss against lizardmen, which tend to lack ASF and don't have high I to negate re-rolls with ASF.
No longer have the pendant of khaleth with its high effective ward save to high S attacks to make an "unkillable" stubborn dreadlord.
Mages no longer innately know the power of darkness spell and the spell is now a signature spell that gives the mage's using +1S and recovers D3 power dice, so it no longer is cheap and easy way to boost power dice.
You will likely see a lot of variation in Dark Elf lists.
One variant is the witch elf horde with death hag and cauldron precisely because the cauldron buffs continue even if the death hag is killed and a 5+ ward on a block of witches is huge. Also, because high armour saves are the biggest bane to witches, expect to see the razor standard on such a unit.
In core, the best choices are:
Witch Elves (either large with cauldron or in basic 10 witch blocks), their madness rule limits the use of characters other than death hags and assassins in this unit
Corsairs (+, ASF plus increased range of handbows and 4+AS in combat makes this a great core unit, smaller units with repeater hand bows used to screen witches or executioner blocks or larger units with two hand weapons even if as command or sac dagger bunker for lvl 4 on foot)
Dark Riders (+. less expensive, gained ASF, and 4+ AS with fast cav, some people will play larger units of these and max core this unit in some armies)
Repeater crossbowmen (-, made more expensive but were uncosted before and still a decent choice)
You will see dreadspears (-, spearmen) and dreadswords (-, sword and shiled) but the mandatory shield and increase in points cost makes them a lot less attractive and they may be used primiarly for a sac dagger llv 4 bunker in competitive lists if at all.
In special,
executioners (+) are great for the points, ASF means that they strike with great weapons at initiativ e order without re-rolling to hit but murderous prowess is effectively a re-roll to wound against T4 or less and KB will ravage characters on foot and standard cav), the T3 and 5=AS and loss of options to protect them is their weakness. Dark Elves discuss using lore of life on lvl 1 or 2 in unit for regen and lvl 4 with life for flesh to stone to protect them; other options include 5+ scaly skin from lore of metal)
black guard (=) are more expensive points wise and retain re-roll to hit plus gain ASF and murderous prowess. That makes this unit quite deadly in combat until it gets up against high T6+ or 2+ or 1+ AS units. The problem is that T3 and 5+ AS and S4 with halberds limits this unit in terms of getting shot up before getting into combat and being depleted in combat absent some decent augments.
repeater bolt throwers (+) with lower cost and S4 AP for 6 shots and S6 no AS and penetrating ranks, do not be surprised if you see DE armies with the max of 4 up to 2999 points. It is one of the few means dark elves will have to threaten high armour save and tough multiwoound models. Daemon princes will suffer with these on the table.
cold one knights (barely +) This unit gained +1 S$ attack by the mount and ASF and murderous prowess for the riders. For that reason, you may still see people run this unit with a dreadlord and/or master for the high mundane armour saves. The increase in points costs makes this less points efficient but worth playing.
shades (+): no change in point cost but gained ASF and murd prowess)-with harpies made more expensive, people will consider this scouting unit as the alternative if not running dark rider units or peg masters/lords. With AHW and GW options and ASF, you may see GWs on these as a specialty unit.
hydras: (-): they got hit with the nerf bat and loss of handlers hurdts their LD. They are still worth their points, most likely with the breath weapon upgrade. They will be used to some extent as a screen but need to be kept within range of lord or at least BSB to avoid failed panic tests and to stay in combat.
Harpies: if they are destroyed, or flee through units, they can now cause panic now and are more expensive with no real gains. Don't expect to see them.
Cold One Chariots (-): +1 attack per mount and ASF and murd p for riders does not compensate for stupidity and increased points cost/, you not see this unit a lot
scroungerunner chariot; not really worth playing. The special harpoon is a threat to monsters but not enough of a threat on a T4 5+ AS chariot for that points cost. You may see this unit occasionally for fun or testing purpoese but I do not expect to see a lot of play.
Rare:
The only rare unit really worth playing is the doomfire warlocks. You can get a unit with two known spells, 4+ ward, s4 with two attacks each, fast cav, ASF and murd prow for not much more than a lvl 3 on foot and their miscast consequences are reduced to D3 wounds on the unit with the ward save allowed. Do not be surprised to see two of these units,, a bit larger unit with a lvl 3 or even lvl 4 on dark steed
Characters: the new lord level characters are really not points efficient relative to dreadlords. You may see a high beastmaster on a manticore maybe. Otherwise, expect to see dreadlords, masters, and sorceresses (lord and hero level). The sea dragon cloak is a 5+ scaly skin save now. This allows a Dark Elf dreadlord or master to obtain a high 2+ or 1+ armour save on foot with just the cloak and heavy armour and a magic armour item and/or shield. It is easier to get to a 1+ AS on a mount.
A common mounts for solo characters is likely to be a peg. This mount gives T4 to rider and +1 wound to a master and fear and flying. With the changes to harpies, a master or dreadlord on a peg is l.ikely to be viewed as essential for dealing with war machines and chariots. Since the peg points cost did not increase and the cloak of twilight gives a character a 3++ ward to shooting and spells + killing blow and multiwounds D3 in the first round of combat, expect to see a dreadlord or master on a peg with the cloak of twlight in army lists. This is called the "dirty harry build". With the cloak being an enchanted item and not a talisman, a dreadlord can have the cloak of twilight with a 1+ AS rerollable with dawnstone on a peg (T4 and no longer exposes to killing blow) and a decent weapon and/or cheap magic armour item. The D3 wounds and killing blow in the first ronnd of combat will make the dreadlord effective in combating monsters and chariots and war machines if the weapon increases the S (halberd for +1S still rerolliing to hit, GW hitting on I, or lance for +2S on charge). The peg gets S5 on the charge as well.
With dark steeds gaining the fast cav rule, it will be more likely to see characters on dark steeds to give them mobility and evasiveness and M9 siwtftstirde speed.
Death hags are only useful in allowing one to take a caudron of blood as a mount. They have no innate ward save or evasive ability absent being in a unit with a cauldron when the gain a 5+ ward save and they can only take one of three gifts of khaine as previously discussed and a magic weapon up to 50 points.
The Cauldron is T6, 4++ ward and 6+ AS with S5 and 5 wounds. The death hag on the cauldron is kind of silly other than its own attacks and allowing one to take the cauldron. If the death hag dies, the cauldron still functions and gives its benefits to its unit of a ward save (6+ ward generally to models in its unit and 5+ to witches, hags, and assassins in its unit, re-roll to wound for all models in units within 6" that have murderous prowess). The bound spell 3 of frenzy or superfrenzy on units with frenzy already is worthwhile. The 5+ ward to witches, hags and assassins in the unit with the caauldron (run much like a plague furnace) is the best use of the cauldron. Given the cost of the death hag and cauldron, I expect to see it run primarily in a witch elf horde unit, possibly with witches brew on the death hag to give the unit super frenzy (2 extra attacks instead of one due to frenzy but -3LD in test to restrain from having to charge). The caudron will sort of put the opposing unit in combat in a bind. The witches with cauldron will deal out so many attacks, rerolling to wound and usually rerolling to hit as to force you to take a ton of saves and rack up combat resolution and deplete your unit such that you will feel presure to get up CR and reduce the witch elf ranks but the cauldron needs to be ultimately killed to take away the ward save and the re-roll to wound ability of the witches and that will take time with it having T6, 5 wounds and a 4+ ward save. Targetting the death hag in combat does very little usually, because killing her typically only takes away her attacks. The cauldron also counts as equivalent to 15 witches in a unit (see BRB p. 98 on characters in unit when the footprint fits within the unit), which means that ten witches (one file on each side of the caudron ) can yield five ranks and when run with 3 witches on one side and four on the other side of the cauldron, the unit counts as a horde (7 witches plus the 60mm wide cauldron). A unit of 35 witches plus a cauldron can, thus, start out as a horde five ranks deep
You might see a death hag on cauldron with a large exec horde adjacent to another unit (within 6" of the cauldron) suich as black guard just becauise of the re-roll ot wound benefit, the 6+ werd +MR1 benefits the unit with the cauldron when targetted by most direct damage and magic missile spells, and witches brew incrreases the number of attacks for its unit and its unit can carry characters in it.
Best of both worlds.