These are the types of things I notice:
*How someone could jump from point A to point not-even-remotely-related - My brain makes weird connections already and I'm good at filling in blanks.
*Words - They fascinate me, I want to know how you speak.
*Colors - as [MENTION=2635]Melkor[/MENTION] said, I pick out things that are slightly abnormal to the majority. I am the first to ask, "Does it bother you that there is a bit of green at the bottom corner of that wall?" when someone shows off their freshly painted living room. I'll probably trail off with something akin to "I understand that the drapery hides it rather effectively, but..."
*Clothes - Tags sticking out, stains, matching, mismatch, mismatching to match, attempt, accident, attempting to make it look like an accident. I can tell.
*Smells - This is especially true when they bug me.
*Someone thinking - For some reason I always catch when the wheels are turning in someone's head and if I know them I can usually guess the conclusion.
*Mannerisms - If you have a tendency to pull on your left sleeve before you tell someone a secret, I've noticed it.
*Imperfections - Whether in my own looks or yours. I know you have that weird scar above your left eye. I'm aware that you have a well placed layer of cover up over the pimple that was beginning to show when I saw you last night.
*Grammar/Punctuation/Spelling - I notice when I do it, even if I may be too lazy to fix it, so I definitely catch when you do. My new thing is subject-verb agreement. I always twitch a little when someone uses 'they' as a singular. Interestingly, I do it all the time. I am most likely a hypocrite, and that person I called out, they hate me.
*Lies - Usually, I know when people are lying. I don't say anything a lot of the time, but I file it away for future knowledge.
*Surroundings/Arrangement - I suppose these two areas could encompass the point I made about color but I'll just keep them separate. I notice what people are doing and how things are placed. It's a common thing for me to put things back into their place/where I think they should go in stores when I'm looking through things. At Barnes and Noble I'm always rearranging the minibooks they have in line at the checkout. Be aware that if you do something strange around me I will somehow see it and snicker silently, but I won't point out your mistake to everyone around.
*Sounds - I eaves drop. I'm sorry.
There's a lot to add but the list seems too long and I have to get to the things I WILL NEVER NOTICE EVER NO MATTER HOW HARD YOU TRY TO MAKE ME:
*Your feelings on a subject - you're going to have to tell me because I don't know whether you like it or not. I'm sorry it's so painfully obvious but I'm at a loss and no, I am not going to be able to guess.
*My things - If something I own goes missing and it isn't one of the belongings that gives me a sense of nostalgia or I hold close then I won't notice that it's gone until I think to use it. I only find this odd because everything in my room is always in the same place and I notice everybody else' things.
*Sounds (Contradiction) - As incongruous as it may seem, this is unconditionally true when someone is trying to get my attention, or talking to me directly when my attention is completely engrossed elsewhere. If I'm reading a book and you say my name, I will not hear you.
*What people think of me - This is in relation to feelings. I don't know, can't guess, don't get it. I am told that there are many people who have crushes on me or that I am liked as a person by the majority. I don't see this.