My Sony NEX 5n is a terrific camera, but the native lenses available from Sony for the E mount is quite limited. Of all the mirrorless systems, here's my quick ranking of the lens packages available.
1) Micro-four thirds
This mount has been around the longest and Panasonic and Olympus have focused on building up a complete range of lenses for these cameras. The availability is very wide, from wide angle to telephoto zooms, a range of specialist primes and quality pro level zooms. There is truly something for everyone.
2) Samsung NX
While I don't like the fact that there is very limited compatibility with Leica lenses for NX mount, Samsung have launched a range of primes that are cheap and good quality. The recently launched 85mm f1.4 was quite a surprise, and its excellent quality levels more so. Not much in the way of pro zooms or specialist primes (other than the 85mm), but I believe there will be more to come.
3) Fuji X mount
Fuji took a very interesting approach with launching only fast primes with the new X-Pro 1. This certainly gave the camera enthusiast credibility, and allowed Fuji to focus on coming up with other lenses. This is one to watch.
4) Sony E mount
Aside from the Carl Zeiss 24mm f1.8, there's not much to get excited about in Sony E mount. This is a shame, as the NEX series are truly excellent cameras and arguably the best mirror-less camera from an image quality perspective. Most of the Sony lenses tested haven't performed well, especially on the demanding 24MP sensor in the NEX-7. However, if you shoot adapted lenses, the focus peaking feature makes it the best of the lot.
5) Ricoh modules
This is system is either very innovative or very weird, depending on where you sit. I sit on the weird side. Ricoh's lens modules incorporate sensors directly into them, rather than into the camera, meaning that a different lens could come with a completely different size sensor. I think this would spin me out when trying to use it. However, they do offer an array of different modules, with probably the Leica lens module being the most interesting.
6) Nikon 1
So far only consumer zooms are available, so not much to get excited about.
7) Pentax Q and K
The Pentax Q and K are truly at the complete opposite ends of the spectrum. The K mount is basically a mirrorless D-SLR that takes Pentax K mount lenses, while the Q is interchangeable lens compact, featuring mostly toy lenses.
It really depends on what you need in a camera. If a wide range of native lenses is very important, then get a micro four thirds camera. If you shoot adapted lenses, then get the NEX. If you want to do both, and don't mind a little extra size and cost, then and the Fuji X Pro1 would be perfect.