2" is about perfect for most tanks. If you have mostly stem plants, 2" is fine. Rooted plants(swords, crypts, etc) do much better with a deeper bed(3-4")
2 inches might be good for burrowing fish, but you'll need to stir it up often to make sure no debris or anearobic bacteria build up under. I always go for 1 to 1.5 inches though.
my 10 gallon has about 1 inch of large gravel. when i set up my 20 gallon, i will go for a finer gravel or sand over a plant-friendly substrait, since i have lots of rooted plants.
Do you guys ever take your gravel out and rinse it off? My Nitrates are outrageous due to slackness in WCs and gravel cleaning...Would this be to drastic a move or remove any good bacteria?