I'd love a spin-off completely about Trixie, but even I see that's never going to happen, so i'm not going there.
Grab a seat, because this is going to be some reading...
Now, I already like the fact she's a unicorn, add the nice color scheme, the boastful character, and the evil laugh, and you've got my favorite pony right there.
I often feel she's looked down upon in the wrong way after the events of the Alicorn Amulet. Clearly, Trixie was sad she was not appreciated at all, so she resorted to the amulet to aid her. However, it was told that the amulet does indeed, corrupt, it's wearer. She wasn't evil, she was lonely and felt like nobody even paid attention to her.
The wizard's hat and cloak are a super-cute factor enhancer, nobody can deny this. Everypony loves a silly hat.
She was a pretty strong villian too, although i admit the amulet did increase her magical abilities. Twilight had to somewhat "cheat" to be able to beat her, showing that Trixie could possibly be nearly, or maybe even as powerful as her.
Also, she's a very honest character, she even manages to trick you that she didn't do it, and with that cute face, you'd believe it easily.
And BAM, i'll give your eyes a rest, for now...