Just got my MT21A today. I've recently bought a few of the MT2a's and have been VERY impressed with them for AA lights, and carry one while working (Fire/EMS), as well as in my pocket as my EDC light, and have another that's attached to my shoulder holster for my HD setup. In fact I was so impressed with them I bought a bunch to give to family members as well. When I saw the MT21a out I figured I'd try it and see if that was better suited for work over the MT2a.
I haven't had a lot of time to play with the 21a yet but here's a few initial unscientific impressions. Right out of the box it had the same "extra's" that my 2a's came with save for a pocket clip. It's almost identical in size to the MT2a with the exception of a slightly larger, smooth head (Batteryjunction lists the head sizes as 1" for the 21a, and .88" for the 2a) with cooling fins and absent the 2 flat areas along the battery tube that the MT2a has.
The hot spot is tighter on the 21a and noticably brighter to the eye. However it doesn't appear to be as "refined" as the 2a's. The 2a's I have all have a nearly perfect circular hot spots with clearly defined edges while the hot spot of the 21a doesn't appear as defined and while looking at the reflector it looks like there may be very slight ripples in it which may be what is causing the difference. The 2a's also have a little brighter spill area as opposed to the 21a but that's understandable considering this light is designed to be a thrower.
User interface is the same for both and I actually like the interface.
Some other minor things about it that 21a that I don't care for as others here have said is the lack of a pocket clip and the new tail cap that doesn't have the protective "wings" like the 2a's do. They're minor differences that to me are kind of big as I like having a pocket clip for all my AA lights, and I also like the ability to tailstand and like having the button protected while it's in my pocket like the 2a's have. The new tailcap seems identical otherwise. Why Nitecore did this I don't know but I don't care for it.
So for final thoughts It's a good, bright, 2xAA throwing light and appears to be just as well built as my 2a's. It does seem to be just a bit less "refined" than my 2a's (judging by the very slight ripples in the reflector) perhaps this is due to initial manufacturing bugs that will be worked out, and I think Nitecore did it a disservice by going with a different tailcap and not including a pocket clip, but it does also cost less than the 2a which is maybe how they justified it. I'd rather pay a few dollars more and have a full featured light. I actually swapped out the tailcap with the 2a from my HD setup so it has the ability to tailstand.