At least about 15 years ago, we really had no idea the long-term effects of donating blood as often as the Red Cross allows over multi-year periods.
I'm negative for cytomegalovirus, which makes my blood suitable for newborns, cancer patients, HIV patents, etc. I used to donate as often as the Red Cross/New York Blood Center would let me (donating plasma in the in-between times when it was too soon to donate whole blood), but at some point that forced me to get off of my red-meat-once-a-week diet because my iron levels were bouncing down around the donation limits.
I didn't realize one check-up with my regular doctor was going to involve a blood screening, so when the nurse drew blood, I told her that I was going to test as anemic since I donated less than a week earlier. I saw the nurse enter my comments into the computer system, but I still got a very concerned call from my doctor a week later about being anemic. The doctor hadn't read the comment on my chart that I had donated recently, but still begged me to eat more red meat and liver and/or cut back on my blood donation frequency. I asked about spinach, broccoli, etc., and she said it was just very difficult to get enough iron that way if I was donating blood that often.
I called up the New York Blood Center to see if my regular doctor was over-reacting and/or they had any suggestions about keeping my iron high while donating as often as possible and still keeping red meat consumption low. They told me that there weren't any long-term studies on people who donate as often as they're allowed over long periods. Also, they reaffirmed that red meat and liver are really much more dense sources of iron than any plant sources.
Anyway, when you donate whole blood/plasma regularly and you're CMV-negative, they cross-check your blood for leukocyte compatibility with immunocompromised patients. Three times, I was asked to donate leukocytes for kids with cancer. Two times I was able to donate leukocytes, and the third time, my hematocrit was too low. I highly recommend it, if nothing else, you feel like a really decent person for a couple of weeks after donating leukocytes to a kid with cancer. It's a pretty low-effort way to leave the world a little better than you found it.
I moved to Hong Kong for a while, and there, they take 400 mL instead of a 500 mL, and let you donate less often, but the minimum body weight limit for donation is lower. (People there are smaller on average and eat less red meat than the average American.) I was also very involved with dragon boat racing there, and being slightly anemic makes training harder, so I only donated once a year in HK. I really should have donated more in the off season.
Now, I've got a young son who's blood-compatible with me, so even though they'd never run my blood directly into my son, my wife would really prefer that I keep my blood in my veins for emergencies. I'd still prefer to donate blood, but my wife really doesn't ask that much from me, and it's a small thing to give up for a few years.