'Privacy' is a word that vanishes from your vocabulary when you and your sibling share a room. You cannot make any late-night phone calls or chat with your friends for long. Your sibling can easily eavesdrop on your secrets, and then you have to bribe him/her to keep his/he mouth shut. You cannot talk in private with your boyfriend/girlfriend as your roomie has his ears only for you.
A big fight usually occurs every other weekend on who will get the room when friends come over. It becomes quite difficult if your friends start mingling with your sibling (especially if you are a girl and you share your room with your brother). And there is a definite apocalypse if both siblings plan to get their friends home on the same day.
If two brothers or two sisters share the room, then there is a constant argument over clothes and who gets to wear them. The boys always fight over which shirt belongs to whom, and the girls bicker on who will wear which dress for the party (ironically, they have their eye on the same dress, always). There are times when they don't find their clothes in their own closet; it somehow always lands up in the sibling's closet.
When you plan to share a room, the sharing is never even. You will notice that your things (which you need the most) are in your sibling's half of the room. And on top of that, your side will be stuffed with your sibling's crap. The result of the latter case being that you are left with less space and more litter to clean everyday.
This is the most prominent case. There is always one sibling who loves working/studying late at night, and the other likes doing the same things early in the morning. The former doesn't let the other one sleep, and the latter spoils his/her sibling's sound sleep in the morning. There isn't any solution to this problem other than to accept and adapt.
Even if you have not responsible for the mess, you are supposed to clean it. Your parents don't care who created it, they only want the room to be clean. The mess-maker skillfully escapes out of this situation most of the time. How much ever you try to explain your parents about who created it, it's absolutely in vain.
A simple notation - 'the tastes are never the same'. If one sibling wishes the room to be painted blue, the other will opt for pink or some non-complementing color. After paint (which parents decide after seeing the big hassle), it's the room decor. There is a serious competition on whose stuff will occupy more space. The room will have the utmost disoriented poster, where a football player might be kicking the ball in the direction of a rockstar who is rendering his hit in full swing.
Along with sharing the room comes the sharing of the TV, computer, etc., which leads to severe squabbles and bickering. Your sibling would like to work on the computer just when you have begun playing on it. There will always be an important show or match which your sibling cannot afford to miss (because of personal interests) when you are engrossed in watching a very interesting movie.
The arguments don't end even when the day comes to an end. At this time of the day, the agenda for the fight is - who turns off the lights! If you set turns for doing the task, the lazier one will definitely pretend to sleep before time when it's his turn. However much you try, you can never win in this situation.
It's actually not easy sharing a room with your sibling, but it's no fun being alone in a room. You will always have a partner in the room. Even if you have a big fight, you will always have a shoulder to cry on when you have had a tough day. You can never be alone in any situation (good or bad). So, for this beautiful bond, a bit of struggle is not too much of a big deal.