I'm definitely not as experienced a competitive player as many on here, but I have been playing AoS since late 1st/early 2nd Edition, so I have been able to follow along with the changes, especially to Seraphon. My very basic thoughts so far:
First of all, 3rd Edition was/is actually quite well balanced, *finally*, so it's a bit disappointing that GW wants to change editions so often. I really think 4-5 years per edition would be better, not 3. Plus, we only got just over a year to play with our 3rd Edition rulebook, which did a lot of things I had been wanting them to do with Seraphon for a long time. Making Saurus on par with other melee troops, new models, etc. But it also heavily nerfed my favorite playstyle which was the Thunder Lizard monster mash army, which was *amazing* in 2e but got gutted in 3e.
I've been skeptical about some of the general changes from 3e to 4e, but there's also some good things. Reinforcement rules in 4e are nice, free endless spells, armies all getting their basic rules at once to help balance them against each other instead of having to wait years for your book to come out to be able to play "properly" for the current edition, and the overall reduction in save-stacking are all positive things.
I'm still a bit iffy on some of the reactions, especially since some armies seem like they will *constantly* be shooting, moving, and charging you IN YOUR TURN. I know a lot of people are excited about more reactivity, but I don't like that everyone seemingly has "trap cards" to worry about when you're moving and trying to do what your army wants to do. I know that everyone gets it, but it adds an unnecessary layer of complexity imo which is kind of weird since with each new edition they always tout how they're simplifying everything to pull in new/younger players. I'd rather save my CP for doing stuff in my turn, not dumping them all trying to shut down my opponent during their turn and knowing that they'll probably do the same to me.
Other than that, most of the changes so far are minor and not really that positive or negative. I guess it's nice that if you choose to take a double turn you don't get a battle tactic that round, but if you're double-turning you're most likely gaining way more in victory points than you would by getting a tactic that round.
I will say that even with GW supposedly getting rid of save-stacking, with everything now having rend and most things having some sort of crit effect on their weapons I hate that it looks like nearly everything in our army will have a 4+ save, which means that in practical terms 90% of the time all our units will be rolling for 5+ saves and will probably die very quickly as a result.
One negative thing people are bringing up that I agree with is the short casting range. Basically all magic we've seen so far has a 12" range, so they really want wizards to be in the danger zone if they're going to cast any spells, and buffs seem to require that a unit be wholly within 12" most of the time, which is also pretty short range. I don't know if we'll get priests back or not, but the Khorne priest preview showed an 18" range on their prayers, which seems a bit strong compared to the near-universal 12" range for spells...
Biggest thing I dislike though is the loss of bravery and the addition of control score. It's a cheap way to "improve" a unit by giving it a buff to friendly units' CS or reducing enemy CS, and they're throwing it on EVERYTHING! I'm not looking forward to constantly having to count up all the increases and decreases to each unit's CS every turn...
As for Seraphon specifically:
Ok, I'm glad the Carnosaur is at least a bit better than the Spearhead preview lead us to believe. The addition of the claws to the weapon profile and the strike first against damaged monsters are both nice. Still fragile as heck with a 4+ save, since everything in the game has rend he'll be sitting on a 5+ save most of the time and will still die to most other comparable attackers or troop units. It's weird that GW seems to want our biggest, baddest dino to focus on picking off wounded things when it should be going head to head with anything smaller than a Maw-Krusha...
Engine of the Gods is at least useable again. Not good, but useable. I guess GW still has trauma from the 2e Engine spam days, we'll probably never see a "good" ruleset for EotG again, or at least not for another couple editions at this rate, lol! Also makes me wonder what they'll do with the Troglodon since it was always in kind of a bad spot right until our new battletome for 3e, I hope it still keeps it's role as a ranged attacking, spellcasting monster. The -1 to hit aura buff is amazing, I really hope it keeps that.
Kroxigors and Saurus look to be doing pretty well, at least they should be able to hold their own vs. other comparable troop types. Hope we at least get a crit effect on our weapons since you know, basic Stormcast troops do mortals on 6s to hit now
Slann warscroll seems pretty good. Hopefully it's not crazy expensive as I imagine he'll still be a near auto-include in most lists. I never liked having to build around summoning so I'm kind of glad that's not much of a thing anymore. The revival roll is nice, but with only a 50/50 chance of getting it on one destroyed unit per turn I don't see it being reliable in most games. Arcane Vassal is still once per turn, boo

That said, I'm surprised to see global unbinds stay, that alone makes the Slann super useful in most situations. Built-in +1 to casting is also nice to see remaining. Losing bonuses to unbinding is rough though.
Asterisms are nice, good to see those come back, even in a weakened state. Pretty solid faction ability overall. just wish the second buff kicked in earlier since most of my battles are already being won or lost by turn 3, lol!
Guess our bites are all gone and our ability to deal mortal wounds is going to depend heavily on subfaction, it will be interesting to see what the other 3 are.
Terradons getting unlimited bombing runs is nice, but having to hug the Terradon Chief to make them useable is not. The auto-wounds on 6s at least makes their ranged attacks somewhat useful. Depending on their cost, I can definitely see them being good ranged support, flying over and bombing things already in combat, getting behind them, and shooting them the next turn before making another bombing run.
Overall, Seraphon seem relatively interesting to play and at least in-line with the rest of the game. Still have mediocre to decent warscrolls, nothing looking particularly strong compared to other units we've seen previewed so far. But then again, this seems to be the edition of "if everything is nerfed, then nothing is nerfed," lol!
Everything sitting on a 4+ save while most other armies' good units are at a 3+ save... I remember when the lack of rend was one of the biggest issues for our army, at least that seems to be better but our lack of saves really concerns me now that literally everything has rend, lol!
All in all, nothing *amazing* but nothing terrible either. Our army looks perfectly playable for the release of 4e, but of course we're only getting a preview of a couple of warscrolls, one subfaction and one spell. It will heavily depend on how good the rest of our stuff is when the indexes officially come out whether I will want to actually play 4e or not.