I really think that class selection will play a much more important role in how many people develop a character. ZOS keeps saying that "it doesn't matter what class you pick, you can do anything - if you want a giant-ass sword, use a giant-ass sword and you can be effective.
I get that - But picking the right class can make you so much more effective. I am going off what I have played at E3, and from what I have seen from sanctioned reviews and such from press-playthroughs.
Let's say you want to play a battle-mage type character... someone good at ranged spells, but lethal once that range closes, emphasizing melee-magic skills over ranged. By starting out with a Dragon Knight or Nightblade, and then going heavily into the ranged magica abilities from destruction staves might be better off than starting out as a pure mage. This way, you have one tree for ranged, and three dragonknight / nightblade as opposed to more ranged, and relying solely on weapon trees for melee.
Class skills make up for 20% of your available skill trees (the number 15 for total skill trees has been floating around) - though in practice, it's closer to 30-50% - chances are you wont be using light, medium, and heavy armor at the same time, or dual wield, 1hand/shield, 2 hand, bow, destruction, restoration.
I am willing to bet that most people will tend to stick to one armor type that fits their gameplay, two weapon sets to switch from ranged to melee, and whichever guild tree fits their play style. So nearly half of their skill trees will be class-based (given that if you're not using dual wield, that tree isn't counted, for example)
If I roll a mage, then end up really enjoying stabbing things in a frenzy of dual wielded daggers, am I as effective, and playing an optimal character build as if I were to re-roll a nightblade? Absolutely not.
----- So many people have said that "your class isn't a 'class' per se... it's more of a starting place, something to build upon to design what you want at the end.
----- I think, as an end-game progression player, that it could be so much more the other way around... sure they're not concrete 'classes' but the skill trees these four classes bring to the table could be much more influential in how some of us structure our character-identity once we hit end-game content.
I guess I'm just a complaining minmaxer.