If one is considering above-the-fireplace mounting, the issues that occur includes the viewing angle, as it causes a sore neck, just like watching in the front row of a movie theater.
Most people find staring up at something for long periods to be uncomfortable.
Secondly, most LCDs look significantly worse if one is not looking at them straight on, according to CNET.
The fix for people who are still set on mounting it above a fireplace, is a wall mount that pivots the TV downward.
"Pointing" the TV toward the sofa is the only way to ensure picture quality with LCDs.
While plasma TVs are less susceptible than LCDs to off-angle degradation, some have screen filters.
There is nothing worse for an electronics product than heat and increasing the operating temperature of the TV can shorten what should be a lively and reliably long life.
Worse, the soot from the fire can get into the TV's innards, doing nothing good.