On a personal note, I'd want it legalized in the US because it does what all those other psych meds I've been prescribed are supposed to do, which is level me out. Instead of high energy, ADHD moments (manic) and low energy pissed/sad/depressed moments, I have normal days, can hold normal conversations with people and order my thoughts in a correct manner. Also I'm not annoyed or pissed off by things that would otherwise bother me, I sleep better and regularly and have less pain in my back and I'm not doped up on painkillers that make me unable to function.
Of course, that is me and it certainly doesn't work for everyone.
On the legal side it will clear out a lot of people who didn't need to be in jail/on probation in the first place other than they violated a law but weren't actually hurting anyone. I've noticed the people who have problems in life "due to pot" actually don't need pot to have problems in life, its just an excuse. "Pot makes you lazy" is a stereotype, not a proven fact. I know a lot of potheads who actually do more when they're high, but thats personal experience talking.
At any rate, I don't see what the big deal is but I get the feeling that most of the people who want it to remain illegal had bad experiences with it and think that because it didn't do anything for them it must be bad or are so uptight that they don't realize the alcohol they drink does way more damage to their body than pot does. There may be other reasons but the general fear-mongering ones I've heard so far are just propaganda.