Negatives to donating blood: You might feel slightly dizzy. The nice medics might make you -- horrors! -- lie down and eat some free crackers and juice. OH DEAR HEAVENS, NOT CAPRI-SUN!
Negatives to donating organs, bone, skin, and corneas, or even your entire face, post-decease: Fucking nothing. As ChurchOfGodBear already mentioned, you are dead at that point and some or all of your body is going to be going away anyway. In almost all of the cases except the face transplant, they can even arrange things so you can still have an open-casket funeral if that's what your relatives need for closure.
Negatives to donating liver lobes, bone marrow, or a kidney while still alive: You have to have surgery. You might need to hang around doctors a bit more for a while.
Positives to donating any of the above: The blood or organ goes into another person. They don't die! They may have complications and will definitely depend on doctors for immunosuppressants, and it's more complicated the more of you they get (e.g. a heart/lung transplant), but they stay alive! The corneas go into somebody else's eyes and the blind can see again... I mean, how much more Jesus-like does it get than that? The skin helps a burn patient, who is in unholy pain -- often through no fault of their own, e.g. maybe a firefighter's gear failed them while they were saving someone else because shit happens -- and even if you are having a skin graft for reasons that are technically your fault, like you're having a racist tattoo removed that is too large or in a bad place to be covered or dermabraded or w/e, it's still of benefit to society not having some guy walking around with a massive swastika on him. But I digress. Someone else who was in terrible trouble of one sort or another that caused them to lose bones or skin, or be in imminent danger of organ failure and resulting death, is saved.
In the US, a year after the transplant, donors and recipients can exchange contact info if both parties agree. There is a weepy but heartwarming video on YouTube somewhere in which an Army veteran describes how he came home, started college, randomly had his cheek swabbed for bone marrow donation suitability by the "Be the Match" Foundation, and saved the life of a seven-year-old girl with leukemia. In the video, they meet for the first time after exchanging anonymized cards and letters. The little girl, who is, as you would guess, alive, clings to the veteran and, for about the first minute of their acquaintance, just sobs all over him and repeats, "Thank you, thank you," because she knew exactly what was at stake and what he gave her and her entire family.
Any motherfucker who can watch that video (or any of the others like it -- this was not an isolated occasion) and still sneer at what marrow donation gave a family and call it a "scam" is not only not a Messiah, but is of dubious humanity, at best.