Leaving dishes in the sink or leftover food out of the fridge (which invites flies), having to buy the dish soap every time, although it's her turn to, are definitely signs of an annoying roomie. Remember the last time he/she had done the dishes without a zillion (yet gentle!) reminders? And yes, hygiene? We better not go there.
Using your stuff, especially cosmetics, and denying that (assuming you wouldn't notice the little water added to it to maintain the quantity); when you actually KNOW it. They use your shoes, socks, and all other accessories, like the sibling you never had. Borrowing your clothes without asking, or easing into your room to check out your stuff, and being very frankly-friendly about it is not always welcome, right?
Roommates can make you forget what a clean bathroom looks like. Ignoring the trash, forgetting the chores, lazing around like bums - basically 'ignoring any household chore' defines a perfectly annoying roommate. In fact, there is more to it. Many might have learned how NOT to share an apartment, either by observing others, or being a slob themselves.
Attacking your stock of food, finishing something you saved for when you get back from work, a simple relish that you may have been dreaming of the whole day - like chips; but, you only find an empty packet staring back at you. Oh, and the best part is that they apologize for it, every time, shamelessly!
Playing their favorite music on high volume, irrespective of your priorities, or the well-before-morning alarms set by them when there is the least possibility of it being useful, because of the snores; you simply go back to your room and try to focus better with the ear plugs on, and grudgingly keep lying in bed, as you cannot go to his room to switch the noise off. Sad!
Adjusting the thermostat according to how cool and hot a human body can feel is like dealing with huge degrees of differences every day. You have to watch a retarded TV show, which your roommate finds to be hilarious! You start to think of how your house is more of an 'accommodation' than your own space. More so when your phone or laptop is pulled off charge just for a 'few minutes'.
You HAVE to be a sincere audience sometimes to anything that might have gone wrong (or that was too good to believe): from a lost earring to a bad day at work, to how beautiful a latest movie is, to career plans like why don't we start up something together. This, despite you having your headphones on and working on the laptop. That is when you truly discover your patience levels.
Using common resources, or sometimes resources of one person being used commonly, either way can be a nightmare. Watching favorite TV shows is planned for, but we know plans are not made to be followed. Familiar? Or, you might have found your wet laundry stacked in a corner, because they had to wash and dry theirs first. And the microwave, which you just cleaned last night, is back with that excellent pasta-sauce-art. The list can go on and on.