I see that your decision on which brand/model to buy leans toward Panasonic. Most of the content that you view have really fast movements on screen like sports TV programs and gaming so you can never go wrong with Panasonic's plasma screen.
I saw this 55-inch Panasonic TV in person and man the picture was uncanny. This is plasma so it really has better picture quality and deeper black levels than LED Edge-lit LCD TVs. The refresh rate is 600Hz so it's really perfect for sports and gaming. Plus it's loaded with MLB and Fox Sports smart TV app.
The extra money that you will shell out will be worth it. Panasonic makes really durable TVs that last for a long time so it is a good investment.
You may want to check out Amazon.com for refurbished TV units if you find $1,400 price point a bit expensive to pay for a really good plasma TV.
Under $1,000 preferable. Could go $1,100 but would have to be a really special set for me to do that. My original benchmark was the Panasonic TC-L47E50 for $750 and it remains my default choice should the price fall to my $700 sweet spot but I want to explore an option of getting a bigger screen as well.
2. Seating distance
Good question as I haven't yet moved into my new place but have seen what it looks like. The layout is here: http://www.fairlanetowncenterapts.com/DocumentManager/center.jpg and the TV will be in the 13 by 11 ft living room. So I would say...around 8-10 ft, probably.
3. Size/placement limitations
Won't be mounting it on the wall for sure. Unless I'm very much mistaken, I'm looking at the 46-55 inch range. The bigger the better?
4. Uses and sources
TV viewing: mostly sports (soccer, auto racing, basketball, occasionally tennis and hockey); movies of the action/comedy variety; some sitcoms and animation (i.e. Family Guy/Simpsons).
DVD viewing: some Blu-Ray, mostly regular DVDs
PS3 gaming: sports and racing games
PC use: occasional gaming (same genres as PS3), web browsing (this is pretty big as my previous TVs often had trouble displaying PC monitor-like images with graphics dimmed and dull even with the PC setting and text not represented in true black). This is actually fairly important, so from what I've been told, plasma is out.
5. Room lighting
One window along the wall, one somewhere in the kitchen (see link for layout). Will most likely have some sunlight most of the time - not planning on blacking the living room out or anything.