David Tennant is a lot lighter than Christopher Eccleston (who did grow on me, but only had one season, so we'll never know what he'd be like later on). the whole series still seems a lot more ponderous than the original run, but at least there's a lot of humor abounding all over. which is part of the charm of the show, it was never quite serious in the first run.
the Cybermen sort of return in this first season, though it's an alternate reality version of them. still, they have never looked more menacing, and I know they return eventually in some battle with the Daleks.
my favorite episode so far is seeing Sarah Jane Smith (the late Elisabeth Sladen) reunite with the Doctor finally. I think this is the first time we've seen this happen since the The Five Doctors way back in 1983. I think a few more companions show up later, but I've not gotten to those episodes yet.
Tennant is also a fan of the show, which is pretty cool. you can see it in his reaction to greeting Sarah Jane, and in the Children in Need clip where he meets the Fifth Doctor.