While size and resolution are important, the most important factor is probably maximum brightness. I've used several DSLR's that had beautiful screens, but could barely be seen in daylight. A big 3", million-dot screen won't do you much good if you can't really see it. Screens based on OLED will usually provide the best of resolution, contrast, and brightness. AMOLED screens are the best type, however I am not sure many DSLR's use them yet (I think a few samsungs do). I believe the Canon 7D uses an OLED screen, which while not quite as nice as an AMOLED screen (less-black blacks), it still offers great brightness and resolution.
Here is a simple video demonstration of AMOLED vs. TFT LCD. In this case, the AMOLED screen is a Samsung...both cameras are P&S, but the tech would compare the same on DSLR's:
OLED/AMOLED screens coming to a camera near you
As far as resolution goes, 920k or 1040k dot screens, which are pretty common on current-generation DSLR's, make it a LOT easier to see how sharp/unsharp or in-focus/out-of-focus your shots are. A 200k and 400k dot screens tend to be dimmer and far less sharp.
Articulating screens can be handy, if you need the functionality. They are often very useful for self-portraits, family/friends portraits that include the photographer, and for shooting video with DSLR's. On the flip side, they are an additional component that can break.