Some players have either quit or moved to Atlantic, so if you haven't played in a while, I'm sure you'll notice a difference.
But keep in mind that there's also a lot more lands today too. People can be spread out all over Trammel, Felucca, Lost Lands, Ilsh., Tokuno, Ter Mur, and out in the open seas with the high seas booster.
One of the best ways to guage how full shard is, is by seeing what the housing situation is.
When I quit UO back in 2007 and i was still playing on Legends, I remember all of Malas being full of houses. When I came back this past November, most of malas was like green acres. No houses anywhere except around the cities.
And when I came to Sonoma from Legends about a month ago, I was shocked when I went through Luna and saw how barren it was compared to other shards. Seeing only half of Luna houses had vendors, and of the half that did, half had not been re-stocked for a long long time. I wasn't use to seeing Luna like that.
That being said, don't let the lack of people around scare you away from here. Sonoma has some of the nicest players on all the shards, a good community, and most people here are willing to help you out any way they can.