Ideal viewing height and angles for LCD and Plasma?

Off angle is a problem when owning a LCd esp. a LED LCd.

Keep in mind that the best viewing experience is when your eyes are in the same position as the center of the screen, all other positions are less.

LCD/LED models have a more limited viewing angle, typically within 30degrees of center before the contrast is reduced significantly. They also lose contrast when viewed from too big of a vertical angle high or low.

Plasma models in contrast have a very wide horizontal viewing angle.
However some of the newer models with a special louver filter (to reduce light reflections especially from overhead lighting) reduces the vertical viewing angle, particularly viewing from a height higher than the screen with similar reduced contrast from that angle only.

So best to have the display mounted so the center is at or a little above typical viewing height, not below.
Check out the viewing angle of different model HDTVs at a local retail store.

I agree with those saying its best if display is eye level (eyes in the center of the screen) I hung my 60" Pioneer Elite higher than that because I thought it looked better in the room (more at a typical picture height) and have regretted it ever since. Next time, I will hang it significantly lower.

As close to eye level as you can manage. You will never regret this.

You want it so that either the top of the screen, or the centre of the screen, depending on your preference, is at eye level. I would be leaning towards having the top of the display at eye level. Mine currently has the centre at eye level and I wish it was a bit lower.

As for viewing angles, it depends on the panel type that an LCD is using. It ranges from being perfectly acceptable for most people (tends to be IPS) to having huge contrast, colour or gamma shifts that look awful at an angle. You need to try the set you're thinking of buying to see if it's acceptable for you. Plasmas are mostly unaffected by viewing angle.

Some of he newer plasma models like Panasonics ST50 and above have a new kind of louver light screen filter that blocks light coming from above, but that will cause loss of contrast if you view it at an angle looking from above the screen.
So for those models, I'd recommend be sure not to mount the panel too low, its OK for the center to be at or above eye level.
Horizontal viewing angle is still great on those plasma models.