For example, if your reputation starts above 50 and then falls below 50, do you lose the ability to leave comments? Or, are privileges forever, once earned?
Can you lose an ability if your reputation falls below a threshold?
Definitely true. I was at 10,009 rep for a few seconds before an upvote was removed:
Not only that, but you can lose an ability if the threshold moves (for example, when a site moves out of beta.) It is clearly checked each time you try to take the action, rather than caching some sort of permissions flag in your profile.
It's true. I had set a bounty worth 50 rep when my reputation was 1049, and I subsequently lost the abillity to see up/down votes (for which you need 1000 points).
It only took me a minute to get the reputation back, though.
Yep. I lost my ability to vote-to-flamethrower(close) when I offered a bounty of 100. Got it back, though :)