What features are important in a wall-mounted LCD tv?

That all depends on what you're looking for. Do you want an HD TV? Do
you want Picture In Picture? Do you want to be able to move it once
mounted?

Personally I would look at a couple things if I were to buy one:

1) HDMI inputs are the newest inputs that support HDTV and I would
want one but you want to look at what you are using the TV with to
make sure it is compatable.
2) The contrast ratio is also a factor (higher the better imo).
3) The size and weight of the TV compared to how far away it will be
and how strong the mount has to be and the wall it is going on.
4) The viewing angle from which you can watch it.
5) The cost $$$.
6) Whether or not it has a built in tuner (built in is nice but not
necessary or even used if you have an HDTV box from your cable or
satallite company).
7) I would also go widescreen since most HDTV and movies are
widescreen and the old 4:3 ratio will be phased out in the future.
8) Obviously you want to be sure it can be wall mounted.
9) Resolution of the TV (again, higher the better imo).
10) Lastly I would look at customer reviews, warranties and all that
other good stuff.

All the above I think are important in a wall-mounted LCD tv.