Two things:
1) Having to find the center of something (block, quilt top, pre-cut) so I can trim it down. Drives me nuts. I never quite get it right the first time so I've taken to cutting it a little too big & then trying again.
2) Pin basting my quilt -- probably for the same reason. That said, I don't dislike the other methods of basting even more. Takes so much time & no one ever sees all that work. I really wish those tagging guns worked better. They seemed like an awesome idea!
Oh, and I don't really like using my seam ripper. I use it vary sparingly. I love it when teachers say that it's better to spend more time practicing/actually FMQ'ing than tearing out seams. I've definitely found that to be true.