Sure, sometimes some people may go a tad overboard with the concept, but in reality, it's not terribly difficult to figure out. As with most such things, it really just boils down to "Don't Be A Dick." I've written before about discussing rape and sexual assault in class, and again, I pretty much just stick to the edict of not being a dick about things, and it's served me pretty damn well. It hasn't required the insane amounts of stress and negotiation that the critics seem to think; instead, it's just required not actively being a dick.
So if not being a dick is too hard, I've come up with a simple and handy way to understand trigger warnings: they're pretty analogous to allergies. In fact, I think it maps really well. Walk through a hopefully non-tortured analogy with me, won't you?
To being with, much like allergies, there are some obviously well know trigger warnings you should be aware of. So just like any decent restaurant should be sure to warn people if they use a bunch of peanuts, what with the fact that peanut allergies are both serious and well-known, you should take a second and think before you bring up news of some sort of horrible sexual assault, because a lot of people have been sexually assaulted and it's a pretty serious trauma for many of them. Or, for instance, maybe don't share a bunch of graphic photos of war casualties to your buddy who just got home from a deployment.
To stretch the analogy further, there are some allergies that are really obscure. If someone is allergic to, say, oranges (I have no idea if this is a thing), they would probably have to get used to letting people know they shouldn't use oranges around them, because most people are probably not thinking about people with orange allergies (if they exist). Similarly, some people may be triggered by very unusual or obscure things, so they may have to go a bit out of their way to let people know not to discuss certain topics around them.
And finally: yes, sometimes people will make a ridiculous fuss about trigger warnings. Maybe even go well beyond what could be considerable reasonable at all. This doesn't invalidate the concept of trigger warnings! It just means that person might be an asshole. It's similar to all the people who have self-diagnosed a gluten insensitivity -- they're just idiots following a trend. But for many people with actual, diagnosed celiac disease, even a tiny amount of gluten can be incredibly damaging to them. Just because some people use the term incorrectly doesn't mean these people are no longer afflicted with celiac.
So again, it's pretty simple. Just don't be a dick. But if that's too hard, just think of trigger warnings as helping people with allergies. You wouldn't serve peanuts to someone with a deadly peanut allergy, so don't trigger people who have experienced emotional trauma.
Update: Shortly after I finished writing this, I came across a post to this article. It provides a much more detailed and in-depth case for basically exactly what I'm arguing (e.g. that trigger warnings aren't a big deal, they just mean not actively being a dick to people). Here's a great pull quote from the linked piece:
Professors give warnings of all sorts that, when not explicitly entangled in the national politics of political correctness, amount less to coddling than to minimizing chances of disengagement with material. “Block off more time this weekend than you usually do, since the reading for Monday is a particularly long one,” for instance, is a reasonable way of reducing the number of students who show up unprepared by issuing a warning. “Today we’re discussing a poem about rape, so be prepared for some graphic discussion, and come to office hours if you have things to say about the poem that you’re not comfortable expressing in class,” meanwhile, is a similarly reasonable way of relieving the immediate pressure to perform in class, which stresses out so many students.