Gaf, do I reroll monk or not? Waffling.
(Was a rogue in vanilla/BC. Was a DK in Wrath/Cata [tank primarily until Firelands])
Personally I'm not a huge fan of the monk archetype, though the one have for WoW seems alright. I probably wouldn't main one as they're almost certainly in for a lollercoaster of buffs/nerfs, and putting up with that can be stressful (as I'm sure you also experienced with DKs during Wrath.)
I haven't tried one myself though, so I couldn't say for sure how good/fun they are. Definitely plan on rolling one at least to 40 or so.
As for myself and switching mains:
It depends on how well they handle account-wide achievements/mounts. If they do it well enough that I'm comfortable with leaving my DK, I'll probably (at least temporarily) retire him as my main. He's had too much put into him until this point for me to really feel good about leaving him with all the achievements, reps grinded (etc.), and just general history.
I'm thinking about going Resto Druid as my main spec/class, probably switching around between the other specs as a secondary. If not that, I may go ranged DPS of some sort to do something I haven't really done competitively. If I don't stick with the DK and do decide to tank, then it's between Prot Warrior or Prot Pally, but probably the Warrior. Just straight melee DPS is almost certainly out though (again, unless I choose to stay with the DK.)