I've been a long-time WoW player, and I haven't had any troubles switching to this game. If you want a faster GCD, MNK is a pretty good class for you. If you are playing a caster, you shouldn't see that much of a difference, since most of cast times are actually on the GCD (with some spells being off GC).

All in all, with cross-class abilities and end-game gear, it's really not too bad. At least, it's really what I do think.