The biggest difference between Stormcast and... well, every other army in the game is the concept of Synergy. The Stormcast are the biggest, strongest units in the entire game with the best stats and abilities out of everyone, I've seen four Paladins take out twice as many Ogres and it wasn't even a close fight. And the Ogres attacked first. However this is balanced by the fact that Stormcast have no real synergy beyond a couple of basic heroes troops. You just fight your opponent straight up and basically as a Stormcast player because of your badass stats, which is what makes them such a good beginner army.
Seraphon, and everyone else really, are much different. Proper use of Seraphon involves pulling off combos with several different units in order to get some extremely heavy buffs, making them as strong as Stormcast. For example, a basic unit may be running a Mystic Shield buff from a mage, a Starlight buff from a Starpriest, a venomous bites buff from the Starpriest again, rerolls to hit from an Astiloth Bearer, the buff granted by a Command Ability, bonuses to charging from a Constellation, rerolls of 1 to wound from an Oldblood, bonus Save and Bravery from a hero, and bonus attacks from an Eternity Warden, with general rerolls from a Starseer, all stacked up into one big combo.
Does that make sense?