Is it okay to disobey a just law just because you disagree with it? For example,
Of course, it isn't morally acceptable to disobey a law merely because you disagree with it (you seem to be confusing the concept of a 'law' with the concept of a 'suggestion'). Let's suppose I'm an American driving in Europe and I want to drive on the right side of the street simply because I prefer it and find the government's insistence that I drive on the left side to be unintuitive and intrusive. This would likely result in someone getting hurt... most likely me. Laws like this one are designed to promote public order and protect people.
I think the drinking age is a good example of this pattern (that laws we disagree with are often there to protect us and promote public order). The drinking age was raised to 21 in the USA largely to cut down on drunk driving accidents and injuries. And (statistically speaking) it has worked rather well. I admit that laws like this one do hinder the liberty of more responsible young adults for the sake of the 'greater good,' but it is hard to argue with thousands of lives saved. In fact, I have a good friend who is a trauma surgeon at the local ER... he says that he would likely be out of work if alcohol were outlawed entirely since the most frequent feature in the patients he sees such as those from car accidents, injuries from violence, etc. is the use of alcohol.... (not that either of us would really like it if alcohol were outlawed).
So the most obvious problems with disobeying a just law are the possible harmful consequences to ourselves and others. There are probably other problems... for example if I break a law that is unnecessary (but not necessarily unjust) I may weaken the authority of the lawgiver and encourage others to break more important laws.