I pretty much completely agree with you on both counts.
While I can certainly understand the desire to change your username (for example you registered intending to only ask a single question, ended up staying and didn't want everyone to know you as iansgasd1234), it's also a pain when someone changes their name and you have no idea who you're talking to all of a sudden. Same actually goes for avatars, but I understand that it's completely unreasonable to have people not being able to change theirs.
If you were to add title changes, I'm sure that would suppress at least some of the expressive urges that drive people to change their names. At the same time, I'm sure it'd be doubly nice to have a post count requirement for this, because then people would feel like it's something that they've earned. 100 posts maybe? 500? As for frequency, you should have something that doesn't have you waiting for ages before you can change your title, but not to the extent that someone can change theirs at the slightest whim. Once every week seems like a reasonable time restriction to me.