Updated August 16, 2012: We think a matte-screen finish offers the best picture quality in rooms where you can't control ambient light.
Most TVs are fine in normal lighting situations, and some glossy screens are better than others. But if you watch TV a lot in a very bright room, or have to place the set where its screen can't avoid reflecting a window or other bright light source, you should strongly consider going to the matte.
Unfortunately, most quality TVs have glossy screens. With LG going glossy on its higher-end LED models this year, the pickings are slimmer than in 2011. Sharp is a standout, Toshiba a pleasant surprise and Samsung uses matte in its lower-end EH lineup.