Know what I hate? First of all, I'm a double sneezer. My sneezes almost always come in pairs. Sometimes more, but rarely less.
I hate when I sneeze the first time and then someone says, "Bless You" before I can get the second one out. I don't want them to have to say it twice, so I stifle that second sneeze. There are few things worse than having to hold back a sneeze. Then I either end up blowing snot all over myself, making some weird noise in my throat (which is obvious to everyone that I was too embarrassed to sneeze and tried to hold it back), or force it out the other end which is never a good idea.
I think everyone should have to wait a solid three seconds before offering up a "Bless You." If you don't, it causes too many problems. Much like that Seinfeld episode.