The trouble is that it always needs to connect to something, so your cable box / DVR / DVD player / games console / etc needs to be housed in somewhere. Usually that’s some sort of boxy furniture that the TV may as well sit on.
You have the problem of cables too – ideally you do this at redecorating time and properly chase the cables into the wall. A line of plastic conduit running down looks almost as bad as the cables themselves out on view.
Finally, most people who wallmount stick them way too high (people who use a chimneybreast for this are especially guilty), you don’t want to be looking up the whole time.
Fine in a bedroom (if you must have a TV in there), with any boxes hidden elsewhere, I can’t think of any wallmounts I’ve seen in a lounge that I’d actually want myself.