The usual hit on micro four thirds format cameras is the lack of control over depth of field. Not that you can't get everything you want (usually) in focus but that you can't drop enough stuff OUT of focus to get the kinds of results you might be used to if you are coming to this format from a full frame camera and moderately fast lenses. The lens I have been pining for is the old Canon 100mm f2.0 which is fairly (but not perfectly) sharp wide open but really seems to be more comfortable when used stopped down to f2.8 or lower. It's a great focal length for my work and there are three contenders in m4:3 that I've been considering. I just initiated ownership of one of these three. It's coming from Amazon.com and I'll be testing it on a series of portrait assignments over the next week.
If it doesn't meet my "stringent" test results I'll send it back and go on to choice number two and, if that doesn't work, choice number three (the choices being order by the decreasing desirability of increasing cost).
I've been on a cheap lens buying jag for the last couple of months and I'm not really slowing down much. I (re-)bought the small and beautiful Sigma 60mm f2.8 DN Art lens because I've always liked the ones I owned before. I like this one as well but even at f2.8 there's more in focus than I would like in some situations.
I thought the Sigma 30mm f1.4 would get me into sweeter territory, and