If you know you don't sightread as well as your colleagues, why don't you work on the piece ahead of time, so you aren't messing up notes that, say, mark a key change, thus making a Wrong Sound and irritating your colleagues? Especially when you've known for a year you're going to be doing the piece? And when you were told the week before what we'd be working on in rehearsal? I mean, everyone makes mistakes. I make plenty. Not everyone has free time to drill a piece over and over ahead of time. But...if you've never been a good sightreader, perhaps you should reconsider. You bring the rest of the choir down with your mistakes. And we want to sing.
Also? If you must tap your foot, it is not necessary to do so as if you're that dinosaur in the Jurassic Park movie. The conductor is allowed to do this if he or she feels the need, as he or she is doing it for all of us. You who are not quite on the conductor's beat? Who is thumping to your own drummer? You can stop now. Your thuds are vibrating through the floor, and irritating me.
In a similar vein, once the choir stops singing, do not take the opportunity to sing/hum/murmur the bit you just screwed up. This means you, and you, and you. Also, you, over there, who thinks no one can hear you. It is irritating, and guess what, usually when we stop singing it's because the conductor has something to say. I am trying to listen, so I might stab you with my pencil if you don't stop, or at least wish to do so. (I won't stab you with my pencil, alas, because that also will disrupt the rehearsal.)
Also, pay attention. Even when you aren't singing. Because any second now you're going to be asked to join in, and it's irritating when you whisper or just outright say, "where are we?" and everyone else is ready to go, and we all have to let our breath out instead of singing.
Last, do not talk in rehearsal. Listen. If you have a desperate question, raise your hand or catch the conductor's eye. Do not chat just because you are not singing. Do not chat while someone else is singing. Just do not chat. Sing.
Here endeth the rant.