I've also been a regular donor (B+) for many years. In the past year the Red Cross has identified me as having an excess of platelets (who knew?).

In a standard whole-blood donation, platelets, red blood cells, plasma, etc. are centrifuged apart and mixed with those of other same-type donors (it takes platelets from three donors to equal one "dose" of platelets for a leukemia or chemotherapy patient). During a platelet donation, enough are taken from a single donor to equal the amount needed to treat the patient. The good thing is that recovery time before donating again is shorter (less than two weeks); the downside is that the donation takes up to two hours, so at the DC donor center, we get to choose a movie to watch during the process!