Yet you can go wild and make so many different builds !

For instance, you can make a priest spec with imp mana burn in disc, mind flay in shadow, and still get the mana/cast time reduction in holy. You'd still be a pretty good raid healer and have a nice toolkit for pvp. Or a hybrid disc/shadow spec with imp mana burn, imp fear+silence and vampiric touch. Or even a holy/shadow spec.

Mage is an even better example with the PoM/AP/Pyro build, deep frost, elemental, pure fure or even arcane. I saw a mage level as arcane with talents no one usually takes, using the arcane missile talents and spirit heavy gear. He was weaving frostbolt+AM to benefit from the 5 second rule, wich meant that he rarely needed to drink.

So yeah, a fair share of talents increased damage by X or the effect by Y but you were free to make the build you wanted, even if it was a bit wonky in raids.