Anything and only things that can be objectively established should be reviewable. This would include some penalties but not all.
Offside? Sure, why not? If the video shows that the guy didn't cross the line early, no penalty. (Allowing this to be challenged would change the calculus of the "free play" for offenses, though).
Facemask? A little trickier, but probably okay.
Pass interference? No. Sure, there are some cases where it could be done objectively (did that big hit come before or after the ball arrived) but generally, it's just way too subjective, and you're just replacing one guy's judgement call with another's. It would accomplish little except for getting people angry.
Another one you can't challenge today, for pretty good reason, is the spot of the ball when a 1st-down doesn't depend on it. The problem there is, what constitutes a successful challenge? If the ball only moves an inch or so either way, sure, the original spot was changed, but is that meaningful?
