Originally Posted by
Rucati
Yeah you'll basically need weapons, trinkets, rings and maybe a couple other pieces, but not the main set and at least bracers, I know that much haha.
As for why there are two sets, I don't really know. I guess so you can mix and match to get the best stats possible. If your class favors Mastery/Crit and you don't want pieces with Haste on them you can do that. For some classes the set bonuses are really bad (Frost Mage set, for example) so it's worth it to just get the ideal stats and give up set bonuses, though I'd say almost all specs want their 4 piece. The fifth piece is usually swapped out.