I was a Rose fan but I never felt a spark between her and the Doctor, to me it seemed more of a paternal or just mentor/apprentice relationship. It was only towards the end, I mean literally the last couple of episodes I detected any kind of love between them, and that was mainly coming from Rose.. in my opinion.
However, from that first second Martha and the Doctor bumped into each other there was a spark for me, and I love a good old bit of UST, so Martha's obvious pining for him is actually something I like abou this new partnership.
Until now, the Doctor always seemed kind of asexual to me, I like this new source of tension. On top of that, I actually sympathise with Martha's annoyance at the Doctor continual mentions of Rose, I feel sorry for her because it must feel like she's never going to live up to this mysterious Rose character and that he'll never value her like she values him.... I dunno, I'm a sucker for this stuff. I can appreciate some fans don't want this additional aspect of lovey dovey stuff, and think it's unessecary, but I love the fact that it adds a new dimension to the show.
Overall, I really really like Martha, I don't know what it is, I can't put my finger on it, I loved Rose but it took a while or her to grow on me whereas I loved Martha straight away.