So, the protection requirement has always seemed extremely low to me. You can't post an answer on a protected question unless you have at least 10 rep, which is one upvote. My default behavior, now that I can protect things, would probably just be to preemptively protect all the things all the time whenever possible-- that threshold is really low and you can only protect a question that's been answered by at least sub-ten-rep user. But if I'm gonna go around protecting all the things ever, I want to make sure that's not a horrible idea for some reason I haven't thought of yet before doing it. Are there situations where the system would let me protect a question but I should let it set unprotected anyways?
You should only protect questions in unusual circumstances.
Despite rumors to the contrary, our site likes new users. New users get rep mainly by answering questions. If new users can't answer questions, then we won't get new users.
Only protect questions when they have "gone wrong." Usually that's some lurid question that is bringing in loads of crap answers from new users seeing it in "hot network questions" or something and coming and dumping junk.
Other than that, don't protect, and we'll quickly unprotect questions that have protection for no clear compelling reason.
Your default shouldn't be to protect questions, so this isn't a matter of when you shouldn't, but when you should. Consider that protection could just always be automatic, but it only is in certain circumstances (e.g. when a question has attracted multiple spam/offensive answers). As Mxyzplk is getting at, one of Stack Exchange's core values is having a bar to entry as low as possible so that new people (including game developers!) have the easiest possible time going from their first visit to answering questions — unregistered accounts are even a thing.
The blog announcement Changes and guidelines for the Protected Question status lays out these guidelines:
Do protect questions that are attracting a lot of non-answers or very poor answers (spam, etc.) from new users.
Don’t protect questions just because they’re linked to on a high-traffic news site like Reddit or Ars Technica. While there’s certainly some correlation between sudden spikes in popularity and associated non-answers, not all popular questions suffer from this.
Do unprotect questions that aren’t currently attracting a lot of attention and don’t have a long history of unproductive answers.
Judicious use of this feature is critical to allowing these sites to handle large amounts of external attention, but over-using it breaks the system: Stack Exchange sites depend on a constant influx of new blood, both to answer new questions and provide updated information on old ones. When in doubt, err on the side of letting new users prove themselves before locking them out.
This announcement also introduced some new restrictions to using the protect feature. It was previously available on any question over a day old, no matter the nature of its answers — but some communities had members which were protecting every popular question or HNQ or everything linked on any news site, and unnecessary protection was doing sufficient damage to our communities that the staff made feature changes to prevent it.
Bear that in mind both that and the fact we can unprotect stuff before you go clicking every protect button in sight.
Myself, I just protect stuff that's accumulating one of our typical spam patterns (witch doctors, vampires, magic spells, etc) because the spammers historically revisit the same questions with special keywords repeatedly and ignore all the others. I also protect questions that are attracting a lot of bad attention, or highly popular questions which over several months or years have kept accumulating questions-in-answers from people who think this place works like a forum. Everything else, I leave alone.