Ok. Here goes.
Peach annoys me. When she was "Princess Toadstool," she was actually capable of doing cool things (see SMRPG, SMB2). Now, she's essentially useless, except as a foil to an overused plotline (ZOGM Bowser/[insert other random force here] captured the princess and we have to find her now [wtd]). Really. Maybe Super Princess Peach will fix this. Maybe.
Toadette, to me, seems like a way to appease the NO[wh]W. I don't know. I always figured (before the character was introduced) that the sex of Toads was indistinguishable to non-residents of the Mushroom Kingdom, kinda like Yoshies.
Dixie, to me, doesn't count as a "Mario" character, per se. I don't know. Her jumping prowess was always a plus for me, especially since I never beat either of the game she was in.
Thus, we are left with Daisy and Birdo. Daisy, many because after SML she seems to me to be the polar opposite of Peach. Since I don't like Peach's personality at all, this is a major plus. Then again. a lot people people think Daisy's personality is annoying.
Birdo is... Birdo, really. Nothing much I can add to this.
But ya, where's the Paper Mario characters?