1) It's a pain in the ass to schedule away rotations...even my friends doing rotations across town got pissed off at the paperwork. Part of it is dependent on your home school, part on the school you're applying, part on whether or not you have someone to stay with while you're there (if I didn't have a buddy who was a resident where I'm going next month, there's absolutely no way I would have gone through the process).
2) Selectivity depends on where you're going and what rotation you're hoping to complete. Timing also matters a lot. Rotations early in 4th year - when people are traveling to make an impression/get LOR's/find out about a particular school or program can have a lot more applicants and then the schools can pick and choose who they'll take, whereas if it's later and there are fewer applicants they can take whoever they can get.
3) Doing well in an away rotation at a particular school can matter a huge deal. At some places, you have little to no chance of matching unless you complete an audition rotation. Most typically these are your ROAD fields at the most highly ranked places, but may also be the case in some other fields at popular residency programs.
4) I've never heard of state residency mattering for match, beyond that it's hard to get into residencies in popular states (see California) no matter where you're from. Being from CA certainly couldn't hurt.
