The main issues around legalisation, for a hypothetical situation, are more subtle than just "the government could make soooooo much money from taxes maaaaan!".
First up, impaired abilities. Weed slows you down, and affects your reaction time and judgement when doing things, such as driving. And don't give me this "I can smoke and drive fine" crap. You're impaired, reaction tests can prove it. Where weed differs from drinking alcohol is that the booze will stay in your system for a short length of time before being flushed or vomited out. Weed can stay in your system for up to six weeks, impairing judgement in a way that can be tested.
Second up, it's physically worse for you than smoking, having much more tar and shit in it. Given that many pot smokers also smoke cigarettes then legalising it may lead to a drastic increase in tobacco consumption, and all the damage and medical costs that includes.
Third up, dosage. Very hard to dose weed, much harder than alcohol. Saying you had half a joint has no meaning when there is no known standard to judge the strength from. For this reason it would be difficult to be able to say how much you had.
Fourth up, "social" smoking. You smoke pot to get stoned. You don't always drink alcohol to get drunk. It differs from booze in this way. People who go home and open a can of beer and drink that beer and have nothing else will have almost no noticable reaction to the alcohol. No one smokes pot in that way - you smoke it to get a reaction, or you don't smoke it at all. That makes it something that should be a very rare thing to do, rather than an every night thing, for most people who aren't artists, poets, rock stars, or possibly animated film animator people who make those really funky twisted worlds.
All those points said - I think weed should be legalised. If you are at home minding your own business and smoking in a reasonably responsible way, then I don't care and you shouldn't be arrested. It should be given the same status as alcohol with all that implies. Drive stoned, lose your license. Go to work stoned, lose your job. However, with respect to weed I don't care if it was something you did a week ago - if your body can't clean your system fast enough then you smoked too much and need to cut back. Legalised pot would not be something you can do every night with no ill effects, it should be something saved for the weekend or even rarer occasions.