I've always loved drawing people more than anything else. It's a pastime in and of itself to fill my sketchbook with characters and come up with histories for them as I draw, determining how it'd shape their appearance, be a result of it, or clash with it. Practicing a variety of body types, expressions, poses, designs, and ages is a lot of fun, and it becomes an exercise in character-building as well as figure art practice.
I love people-watching too, but don't tend to draw them in public. I'm more likely to stare at you intensely when I think you're not looking, then file away your idiosyncrasies to give to one of my characters later. :]
On the flip side, I'm not a fan of drawing anything that isn't a person, and definitely not of anything inorganic or manmade. Used to be into drawing animals as a kid (lots of horses, birds, deer, and wolves), but at some point it started to bore me. Can't draw good building/interior settings, vehicles, or techy sci-fi stuff (ex: robots) to save my life.