Miter saw advice and recommendations
There are a few different things to look at:


Compound vs. Non-Compound - This refers to the kind of cuts you can make with it.  A compound saw can both swivel and tilt.  A non compound saw usually just does one or the other.  Almost all miter saws are compound, and that is probably what you want.
Sliding vs. Non-Sliding - A sliding saw will let you cut bigger boards (2x10, 2x12, etc) but will be more expensive and quite a bit bulkier.  I have a sliding saw, but honestly find that I hardly ever use the sliding feature.  How often you use it will depend on how often you work with larger boards (2x8, 2x10, 2x12)
10" vs. 12".  A 10" saw is good enough for most work and less expensive, but can only cut up to about a 2x6 in a single cut (unless it's a sliding saw).  A 12" saw will cut 2x8 boards in a single cut.  You can also cut 4x material easily on a 12" saw, while a 10" saw won't quite make it all the way through.


My suggestion - if you plan on using it mostly for trim or framing work, buy a 10" compound non-sliding saw.  If you plan to use it a lot for decking or outdoor work with larger boards or 4x material, get a 12" saw, maybe sliding if it's in your budget. 

As for quality items, here are a few things to look at:


Does the miter gauge look accurate? Is it easy to read?  If you adjust the saw, can you easily set it to within one degree of accuracy?
Once you adjust the angle and set it, how tightly does it lock in?  If it feels "sloppy" you'll have a hard time getting nice cuts.


Brand wise, DeWalt makes some really nice ones, as does Hitachi.