Mostly the pain of the needle, the time involved in the donation itself, and potentially dehydration if you don't drink enough that day. You should also avoid things like alcohol or drugs around the donation time so that could be a "problem" if you drink or do drugs. Aside from that, there's not really any "down sides" and most people I've talked to appreciate the extra money. I hope I helped!
: I almost forgot, the places where you can donate plasma have a lot of safety protocols in place to avoid any risk of infection or contamination, which is why I say there aren't really other down sides. I've blood or plasma donations dozens of times and I've never had an infection from it because of the cleaning protocols and care instructions afterward.
Health/nutrition classes and personal experience