I'm going to have to massively disagree with that. Have you played an arcane warrior before? You go from DPS mage to ridiculous tank with shit loads of sustainables. Couple with spirit healer to become a walking health dispenser or blood mage to be able to cast magic from constitution (to very different forms of play)
most of the specializations ARE game changing ... with SOC and letahlity you can pump into cunning instead of strength, making up for the reduced attack rate of low strength)
You seem to be taking an incredibly liberal definition of "severely effecting gameplay". Adding an extra ability or two, giving a minor attribute bonus, and maybe changing your auto-attack speed does not dramatically change how you play a class. Reaver and Berserker play basically identically to any other DPS warrior with new passives that make them harder to heal/do slightly more damage, as does Champion with a tank or DPS warrior, just giving you a new button to hit with War Cry. Templar is much the same, though at least its new button is Ranged, which is new.
Rogue is much the same, Assassin and Duelist give you new abilities and passives but you're still using the same base rogue melee game, Bard can work for either ranged or melee rogues but you're still playing the same game with a new passive. Ranger is the most game changing since it gives you a pet, but it doesn't really change anything either; you're just doing what you were before with a furball doing damage beside you.
Mage gameplay is definitely the one that changes the most (which isn't a surprise since Mage gameplay in general is much more varied), since Spirit Healer gives you some much needed variety in healing with an aura and an AoE, Blood Mage lets you cast off Con, Arcane Warrior makes you a hybrid of sorts (although most people just play it as a warrior with ridiculous mitigation and cut out magic almost completely with the passive stacking), and Shapeshifter makes you wish there was a way to respec your character.
I never said that they were irrelevant since they do give you some benefit and some are obviously more ideal than others(with Spirit Healer frankly being almost too good), but adding like two buttons is not even close to game changing for something that is presumably supposed to be a major decision in developing your character. There's still combinations that are better than others (obviously you need a tank, bard is helpful in caster heavy parts for the mana regen, etc.) but the advanced classes play almost identically to their base counterparts with an extra button added in, which is lazy at best and one of the few major weaknesses in this game that I suspect will be improved dramatically in the sequel.
Also cunning rogues are terribly suboptimal unless you're playing on PC with the dagger patch (in which case they're probably at least on par if you're controlling them but the AI doesn't flank very well).
I would agree with not worrying much on any readers' first playthrough about building optimal characters or party, it's pretty necessary on Nightmare (especially if you opt not to use some of the more broken shit like Force Field), but on Normal or even Hard it isn't too important. A lot of the fun of this game is playing it how you want it; the minmaxing can wait until you've explored a bit, I'd think.
As far as tips for new people
-While most classes will get it to the minimum required for the diplomacy skill thinger near-automatically eventually, it's not a terrible idea to boost Cunning to 16 early on just to make sure you can take it every time you have the chance... being able to have as many dialogue options available as possible is really helpful just to make sure you're able to say what you want to say.
-Talk to your characters! Mostly because they're interesting, but also because they tell you a lot of stuff that seems completely random and useless but usually isn't. A lot of them hint at gifts that are important to them. Use gifts, by the way - you're rather unlikely(read: impossible) to cap out your approval with all your party members through dialogue alone and they get absolutely incredible bonuses when you're +100. This is doubly true with people like Alistair (Constitution) and Morrigan (Magic) who have inspiration boosts to their key stats (not everyone does for some reason...)
-Teach Morrigan Heal oh my GOD. Seriously. It's only one point. You'll thank me later. Besides, you never know when you'll accidentally kill the party's only spirit healer forever on consecutive playthroughs. Cough. Yeah. Be more careful with potential party members than I was round 1...
-Do the sidequests! It isn't hugely essential since outside of the final events enemies scale based on your level, but you sidequest are worth at least a fifth of the total xp, offer pretty good rewards, and for the most part are pretty interesting compared to quests in games like Fable where it was obvious filler content.
EDIT: Also Sten is a douche. He always likes me by the end but I'm still not a fan. It might be his gruff monotone, his lack of a starting advanced class thing (at least Morrigan has SOMETHING even though she _practically_ starts without one...), but it's probably just the fact while he is merely a decent character every other character is spectacular, and for the most part could each be the best party member in a game of their own if they weren't all competing with each other. Alistair, Morrigan, Zevran, Leliana(though I wish they'd laid off on her accent a bit), Shale, and Ohgren are all just great.
EDIT2: Also as far as "reasons the game is great" it dawned on me none of us mentioned the music. It's not something you'd see from, say, Nobou Uematsu, but it plays out like a movie score and it's very fitting and effective. You don't notice it very often, but it helps grab you enough that you're not listening for it because you're immersed in the scene.