It's kinda repetitive after you learn all your skills and run all the instances. The mobs behave according to certain rules i.e. attack highest ergo.
But the game itself is HUGE in term of things to do. It's so much deeper than anything other game I've played. It would take you years to experience all the class builds, all the spec trees, and professions (that is you just level to cap). And mostly, the game is very well designed. The only part that sucks is walking from A to B in lower levels (and being ganked or being a warrior lol).
The graphics rocks because it doesn't idolize skinny, large blossom chicks or perfectly proportioned hunks. You can look at them like the abstract of your personality.

They have very cool animations. In general, it's the best that anyone can make without requiring a high end graphic card to run. WoW requires very little hardware power.
To answer your questions:
Different class (e.g. warrior, priest, hunter, rogues, warlock, mage, shaman, druid, paladin) have entirely different spells. Each class can be personalize with different spec trees, which strengthen you in one discipline or another. 1 level = 1 talent point to spend from level 10 on.
Every class has their specialty. Hunters get to tame beasts like wolves, spiders, snakes, bears... to help them. Warlocks and mages can summon someone to fight with them. Certain class can heal other can.
The key in WoW is to understand your role and co-operate. But of course, you can choose to grind like mad if killing 10 of this/that is too hard for u.
You can have a lot of chars, at least 8 on each server I think. Thing is, if you're on a PvP server (i.e. Kill on sight server), you can't have char on opposed faction. That is, you can't have a horde char and an alliance char too.
Yes there's night time and day time. It's magnificently done.
But do yourself a favor, don't be an ass if you play WoW. Selfish misbehaviors like ninja looting, begging are heavily frowned upon for lack of a bitchier term.