I dislike it when people don't read all the posts, but I'll admit I've been guilty of this.

At one point I was just busy and didn't want to be bothered with RPing, but I still made an effort so as to not ruin the experience for others.  But I only read the posts pertaining to my character and his situation.  I just had other things to do than read several other posts.

I've also stopped reading a person's post simply because of the way they typed stuff.  It was annoying and hard to figure out what was going on, and reading it would just confuse me so I ignored all of his posts.

I'm a little better at things now, and I try my best to read posts as they come so as to not get overwhelmed, and then re-read them if I ever have time.