Depends on the server you play on. Ranges from populated to nearly barren. As I only play with friends, and would rather be shot than involve myself in a Blizzard games community, I went for barren.
It's fairly buggy as well, but nothing that wont be ironed out in a month or two, and nothing game crippling, mostly annoyances.
My only complaint is the latency meter. I swear its fucking broken. 25 people surrounding me in Ironforge acting like fools = 50-100 ping, farming in another area with maybe 1-2 other people, random, unexplainable jumps to 300+ ping.
It never lags, which furthers my broken theory, but that goddamn bar randomly turning yellow for no reason drives me nuts. Wish I could turn it off.