As far as MW's go, they are really different from other healers to play because you have to balance 2 resources. Paladins holy resource is more for bonus heals, their mana heals are strong as it is already. Monks however pretty much need their chi heals for throughput heals. Plus there are a few ways to gain chi. Faster and mana efficient through melee, ranged healing is slow but the ranged heal is a little safer, Spinning crane kick for when your whole group needs healing but also need that chi heal on the tank pretty soon which is not mana efficient. You can get away with just doing the ranged heal for chi generation but to maximise the potential you need to swap mid fight between different ways. So besides staying out of fire you really need to stay aware of everything and not just your Grid (or whatever addon).
And their healing style doesn't really have the standard flashheal, big slow heal, mana efficient heal, AoE heal, o-shit heal. This is more a thing you need to adjust to in the beginning though.
So overall they are not the easiest to play. One of the more difficult healers and for sure the most stressfull because you can be at 0 chi right when the raidgroup takes a big hit, just when you need your chi.
Their tank healing is not their strong point but enough. Any healer can be tankhealer. It is just that monks are very much OP in raidhealing at the moment. Without trying you go 45% of total healing done in your 10 man-3 healers raid, Heck even in LFR I got 40% on bosses with 5 healers. Only if there is another monk, there is some competition. I have played healers since vanilla and I don't think there have been many times where other healers were doing this. Druids on specific fights in WotLK were OP. Shamans late in TBC probably.
So why would any guild use them as tank healers?
And people saying they dumped their MW because healing was boring at lower levels. Did you ever roll another healing class? All the way up to 80 you are pretty much doing nothing unless you let the tank go DPS spec and use the mage as tank. Seriously, those instances are so undertuned, it is not even funny. And after 60 it gets even worse mind you because then all the tanks have healing as well. If anything, monks are still fun because you can DPS the whole time and that little healing that might be needed, you do automatically through your fistweaving.
Heck, now I am a lot more geared and if a Blood DK joins a HC that is somewhat geared as well, I don't have to do anything at all. I think the HC's are even worse then WotlK atm. If you aren't PVP-ing (they are average PVP healers, a little better in BG's, fairly weak in arena's) or raiding, there isn't much to do for you healing wise.