My cat must be the sweetest thing in creation - she's very loving, and perches next to me all of the time when I'm using my PC or Xbox.
The only things that I find annoying are when I'm trying to sleep she insists on sitting next to my head purring very, very loudly.
She also, after using her litter box sounds like she's tunnelling to Australia - the noise she makes can keep me awake for hours.
The third and final annoying thing she does is 'talk' to the ghost of a grey cat that inhabits this building. I haven't seen it personally, but my cousin has, and a quite a few people who've stayed over had told me about feeling something lying on their bed with them. Can't say positively that it's not my cat there with them as they lie in bed though.
She often behaves quite dog-like too, fetching the mail from the front door for me and such like.