Solarflare has some good input on mirrorless.... Micro 4/3 would be my recommendation among those: Either Olympus or Panasonic. Olympus uses IBIS while Panasonic uses In-lens IS. The lenses are interchangeable, numerous, AND there are so many bodies to choose from.... there something from everyone at all sorts of budgets.
If I were to choose micro 4/3 with a modest budget.
1) Olympus E-PL2 w/ 14-42 kit
= (refurbished @ cameta) $279USD
2) Panasonic 14mm f/2.5
=
ebay commonly brand new for under $200USD (My co-worker got his for $170)
3) Panasonic 20mm f/1.7
= BH or amazon for $360USD
4) Olympus 45mm f/1.8
= BH or amazon for $400USD
Those are the three high value prime lenses + a kit zoom.
* Alternatively, you can check out the Panasonic G3 instead of the E-PL2.
* A better wide angle prime... check out the Olympus 12mm f/2
* A better normal lens.. check out the Panasonic/Leica 25mm f/1.4 Summilux
* A macro... check out the Panasonic 45mm f/2.8 macro
* A good quality wide angle zoom... checkout the Olympus 9-18mm
* An even better quality wide angle zoom... checkout the Panasonic 7-14mm
* A long telephoto (a bit slow aperture)... Panasonic 100-300mm
* A medium telephoto... Panasonic 45-200 or Olympus 45-175.
* A fast pro-ish zoom... Panasonic 12-35mm f/2.8
... and the list goes on and on and on....
Four Thirds | Four Thirds | Micro Four Thirds | Chart(Lenses)
Not to mention third party, like the voigtlander 25mm f/0.95 or any number of lenses that can be adapted.
As Solarflare mentioned, the biggest disadvantage to this system is its ability to track moving subjects (ie sports).