I am a PhD student and hate it here. How can I warn prospective students during admit weekend without ruining my reputation?

• Focus on facts, rather than your emotional impressions.
• Don't try to overgeneralize. Say what didn't work for you, rather than imply that it does not work for everyone (unless everyone agrees) or it won't work for the prospective student (who knows...).
• You could say "I would have chosen differently" (ideally, adding the other possible options) or "overall, I dislike this program" rather than "this program sucks" or going into long rants.

lack of funding,

Here I think you can be straightforward. Probably the faculty are just as disappointed as you are.

administration being uncaring towards students,

I don't know exactly what this means; as a grad student I never tried to attract the attention of any administrator. If you have any specific problems you might bring them up.

general departmental atmosphere and culture among the grad students, general culture of the school,

If you're talking with an old friend privately, sure, bring it up. If you're meeting students at some big admit weekend, then you would piss people off by saying anything, and indeed there's not much reason for you to: prospectives will get a chance to observe the atmosphere for themselves.

poor location, etc etc.

Griping about this is pretty common. (Among faculty too!) I don't think it's especially frowned upon to complain about this, at least if your point of view is widely shared or if you are speaking to prospectives in private.

Indeed, I remember one student at one school telling me: "The only positive thing I have to say about the campus or the location is that there is adequate parking."

Also my school is the only top school in my field that has a "rotation system" for picking advisors, and that is generally framed as a benefit, and a reason to choose the school.

Tread a little bit more carefully here. "In retrospect, I believe that I would have fared better without this system; my experience was blah-blah-blah. But this is a system that our school touts, so you might talk to other people too, to get a sense of what would be best for you."

That said, if this were some kind of admit weekend, I'd consider inventing an excuse to be elsewhere for the day. If not many current grad students are willing to show up and talk to prospectives, that will itself serve as the warning you want to offer.

I personally do not think you can warn perspective students without ruining your reputation at a departmentally sponsored event. If your department asks you, or even just provides an opportunity, to meet with perspective students, the expectation is that you will represent the department in the best possible way, while being honest. That means you should not volunteer things that you do not like. if you are asked a question about something you do not like, you should of course answer it honestly. Using such an event to express your views about the short comings of the department, is unprofessional and I would think less of a student who behaved that way. If a potential student contacts you unofficially, then it is less inappropriate to, but still inappropriate in my opinion, to disparage your department.