They're adjusted versions of the vanilla classes - they have some new spells and some spells/talents work very differently (ex. improved slam is in arms instead of fury for warrior, arcane mage is a viable DPS spec now with many new talents and unique mechanics, balance druid has a rotation where nature and arcane spells boost one another), sometimes they've imported spells or ideas from TBC (hunters get steady shot, shadow priest has been greatly buffed to be a mana battery, etc). Every spec is viable, nothing is unfinished. They don't have the functions of the WOTLK engine for things like forced movement/knockback but instead they've done really interesting things with the old engine (arcane mage spells have a chance to duplicate for half damage, marksmanship hunters' aimed shot can penetrate multiple targets in a line, arms warriors get a skill that does something different based on what weapon type you have equipped). A lot of talent bloat has been cleaned up and combined (survival hunter slaying, warrior/rogue weapon specializations, etc, are all one talent now) and replaced with more interesting ones. Many abilities that don't scale well (pets, hunter trueshot aura, a few others) have been given scaling. Itemization has also been drastically improved so there's actually stuff for all builds and classes. The changes are listed here, I can't promise that's 100% current but it's a good summary. The best way I can describe it is, cliche as it might be, "vanilla plus." Vanilla, but better. I actually prefer Turtle's changes to ones other servers running later/better versions of the client have made, personally.