I am a PhD student and hate it here. How can I warn prospective students during admit weekend without ruining my reputation? On the one hand, I want to warn students not to come to my program.  On the other hand, I don't want to be "that one," and I also don't want to tarnish the reputation of the specific people I worked with (who I mostly like).

More specifically:

I want to warn students about lack of funding, administration being uncaring towards students, certain PhD requirements being a lot more onerous than they say they are, general departmental atmosphere and culture among the grad students, general culture of the school, poor location, etc etc.

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.  But I don't think students really realize how much rotations suck until after they've come here.
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.