Traded my D90 to an EP-3 and now starting to ask why I did it.

Some thoughts (no particular order)

You need to give yourself some ADJUSTMENT TIME

• going from a DSLR to any mirrorless.  Olympus E-P3 is quite fast in single shot AF.  With practice, you can actually shoot single shot AF just as fast...if not faster...than most DSLR.  E-P3 can also do 1Touch-Screen-Focus, which I find it faster than Center & Recomposing using DSLR

Software Noise Ninja bridge the High ISO gap

• For iso1600 ~ iso6400.  Noise Reduction software bridge the DSLR gap quite nicely

Its a Good TRADE, I would do it

• I consider your D90 to E-P3 LUCKY!  Let's face it, D90 is a 2008 camera with limited 720p video support.  It was a great camera at its time, but it is showing its age.  Olympus E-P3 can also function as a 1080p video cam with AF, has Touch-Screen, more AF points, smaller, and image stabilization in body.
• If someone present me with the same deal, I too would jump on it, so don't regret your decision

My only criticism against m43 is Lens Prices

• Fast Prime on m43 is expensive
• Fast Normal Nikon 35mm f/1.8 (50mm equivalent) is just $199
• Panasonic 25mm f/1.4 (50mm equivalent) is around $500 ~ $580

• Nikon also enjoy a plethora of cheap 2nd hand prime for really good price

Other than that m43 has almost no weakness at this time.  Hopefully the rumor Sigma 25mm f/1.4 will bring in some competition and lower the lens price.