There is a disadvantage due to the sensor size, everyone will say that. While true, it is actually quite minimal. Obviously, it varies between models but a recent m4/3 camera compares to a recent one with an APS-C within one stop in terms of noise, and it is rarely visible until ISO 800+. Here is how the differences looked last year. All these models had one successor since.
The only serious difference is actually speed. All the current m4/3 cameras use contrast-detect AF which is slower by a noticeable margin to phase-detect, which is used by DSLRs. There is a small but noticeable display lag as well because the LCD or EVF works electronically (not a the speed of light as with OVFs). With subjects that move slowly this is not a problem at all, but for action it can be.
There are obviously more differences but the remaining ones tend to be more specific to certain types of photography. Certain features are not available yet in m43 too, such as weather-sealing, sync-port, GPS, support for AAs, etc. You'll have to investigate which ones impact you depending on your photography style.