The de-criminilization of marijuana is a very 'odd' situation for lawmakers and society. As a former daily user and now a rare 'once every couple of months' user, I understand the ups and the downs associated with the drug. With my B.S. in Forensic Chemistry with a focus on drugs of abuse, you could kind of say that I majored in the stuff in college.
The biggest problem with pot is the utter lack of organized, scientific research. Sure there's a lot of 'I know so-and-so who smokes pot daily and there's nothing wrong', or 'I know someone who smokes every day and his/her life is in shambles' stories, but there is no organized research. The majority of the research was performed under government grants in the middle 20th century where the government pretty much said 'tell me that the stuff is bad or you don't get your grant money'. There has not been a helluva lot of research done on the stuff because of all the strict rules and licensing the government forces you to go through if you want to perform research. For many institutions, there's just not enough of an economical benefit to research what pot does to you.
As a human being, I wouldn't feel comfortable with a sudden all out legalization of something when we're not 100% certain on how it affects you. Again, the only evidence we have is a little bit of heresay evidence. It's much easier to keep something illegal than it is to make it legal, suddenly find out that it's horrible for you, then try and make it illegal again.
Marijuana for smoking was first introduced to the United States by Mexican and Carribbien(sp?) immigrants in the early 19th century, I believe. At first the American public really didn't think much of it and its use wasn't all that widspread. Slowly, however, the use of marijuana started to increase tremendously. The wealthy white owners of businesses who used these immigrant workers became concerned that their work production would suffer from all of the pot they were smoking and that it would be a negative influence on their other workers. Because of this, taxes were passed which put a heavy taxation on the marijuana use. This prevented the poor citizens from being able to afford to smoke pot. The wealthy could still use it if they wanted to because they could afford the taxes, but eventually the tax level became so high that it was just not economically feasible to smoke pot. So pot wasn't initially outlawed in the US. It was just taxed beyond belief before eventually falling under the Drugs of Abuse classification.
That happened in the VERY early 20th century I believe, and it has been considered 'illegal' ever since. Some research has been done on pot, however, and some facts are known. First off is that the LD50 (The amount of a substance needed to kill half the population it's given to) for Delta-9-THC is INSANELY high. You need to ingest an obsenely large amount of pot in order to die from it. It's an amount that could only happen in rare instances when people were purposely trying to kill themselves. It's not like alcohol where you can kill yourself without realizing it.
Marijuana causes an increase in appetite by slowly lowering the blood sugar. It makes the body more sensitive to levels of insulin in the blood and causes the cells to use that insulin to 'burn' sugar. This results in a slow lowering of the blood sugar which helps stimulate the appetite.
Marijuana stimulates ones appetite. 'The Munchies' are one of the main reasons why synthetically produced THC is marketed as the drug Dronabinol (Marinol) which is used in some chemotherapy regimens to help the patient keep their appetite. The fact that the FDA has approved Dronabinol/Marinol proves that the DEA is completely hypocratic because the DEA has classified marijuana as a Level 1 Drug meaning that it has 'no medical benefit and has not been assessed for safety data'. (In order to get FDA approval on a drug, a copious amount of safety data is needed. It's because of this need for safety data that I have a job as a clinical data analyst).
Marijuana alters the brain chemistry and can have a profound affect on ones ability to remember things. For some, their memory isn't affected at all. For others, they suddenly can't remember anything. It's something that should be further researched, but the predominating data shows that it does alter how one remembers things. I know that when I used it my memory was totally shot and it has taken quite a bit of time for me to recover.
Marijuana lowers your blood pressure as well as your intraocular pressure. Hence why it is a very effective glaucoma treatment.
Heavy marijuana use causes a lack of motivation. Like all things, if used in moderation this side affect can be avoided.
There are many other things which are already known, but as it's getting late I'm a bit tired and don't want to drag this post on much longer.
My personal belief is that pot needs to first be further researched and slowly de-criminalized. Don't just go out and say 'it's legal to smoke pot'. Slowly relax the restrictions on it by taking it off of the list of tested for drugs. Then make it legal to use it. Then make it legal to posess small amounts, etc. etc.
Now someone else in the thread said that "Personal Freedom" should allow them to use any drug they want in the privacy of their own home. However, one thing 'Personal Freedom' doesn't allow you to do is infringe on my personal freedoms. Things are made illegal because their use affects not only those who are using them, but those who are around them. What about the parent who decided to go shoot heroin instead of paying for their kid's visit to the doctor? How about your babysitter using cocaine instead of showing up to babysit your child? There are many more cases where someone 'doing something in their own home' severely affects more than just themselves. You can't pass a law saying 'you can do anything you want as long as you're single and don't have kids'. That's just not feasible. Whether these are laws we agree with or not doesn't matter. Society as a whole is far more important than any of us are as individuals. In 100 years, none of us will be on this planet, but the decisions we make and the things we do will affect the lives of those who are here. Society is never ending, individual humans are. So think about that the next time you cry out about your 'Personal Freedom'.