First of all I believe that Micro-Four-Thirds is the way forward for all amateur and some pro photographers because the technology has reached the good-enough point for it to overtake the APS 1.6x crop sensors in DSLRs. I expect these to die out sooner or later, leaving just MFT and full-frame DSLRs (pretty much along the same lines as Trey Ratcliffe)
The ability to carry a bag of prime and zoom lenses weighing less than a single APS UWA zoom is a benefit I can't ignore, as someone that likes to get out into the mountains.
I finally decided my Panasonic G3 had to go when I was up on the mountain a few weeks ago, and the shutter release failed to work again. The micro-jack style connection seems to be prone to oxidation build-up causing a flaky connection - which whilst you can use contact cleaner to restore it - isn't my idea of a reliable system.
The second trigger for me to want to change cameras was the fact that I was using manual and aperture priority mode more often than I had in the past, and the tiny thumbwheel on the G3 was beginning to annoy me. The difference in force required to rotate the dial and push the button is so slim that I constantly found myself switching from aperture to exposure when trying to change one or the other.
This wasn't such an issue on the G2 as the thumbwheel was more substantial - which you can see from this comparison image from the dpreview review of the G3.
The OM-D on the other hand has 2 large, tactile dials which can be configured to drive a variety of manual controls, making it much easier to adjust the exposure setting you want to tune. You can see more images on the dpreview of the OM-D E-M5.