Smaller pixels which gives more detail. Also relates a little to viewing distance. When you watch your HD tv you'll be a few metres from the screen so larger pixels won't be seen. With a monitor you'll only be 2-3ft away so larger pixels would be far more noticeable.
The dell will be able to display HD but will have to stretch the 1900 odd pixels to match the dell's 2560 resolution.
well the samsung's is quoted dynamically which isnt a true representation of ifs real contrast ratio. its a bit like quoting PMPO for amplifiers instead of RMS - its pretty meaningless.
for dynamic contrast, im pretty sure its as simple as quoting the darkest shade the set can display vs the lightest shade, which is fine expect for one problem - you canT display both at the same time! due to the backlight and other things, you have to make a compromise. at night, i find i HAVE to turn the backlight on my sony 40w200 right down to 1 (lowest it'll go), otherwise the black performance just isnt good enough. this of course lowers the white performance - you cant have both but at night, its bright enough anyway. conversely in daylight, i have it at set to 7 (out of 10). black performance suffers but colour and white levels are much better during the day.
As an example, my 40w2000 is quoted at either 6000:1 or 8000:1 dynamic range (i forget which). but in testing, they actually churned out around 1050:1 which while much lower, is still very good (and better than any pc monitor too, which i can confirm. 24" dells, my 24" ocuk DGM, various other monitors....none of them are as good as my sony side-by-side).
Now, most tv's do have a trick bit of software to alter the backlight and things on-the-fly to achieve that dynamic ratio, but you'll find that once you realize you can see the backlight going up and down and the contrast constantly changing, you'll soon get annoyed with it and turn it off.
on short, anything quoted as dynamic contrast is marketing tosh