What is the difference between current street pot, pot from the 1970's, and medical marijuana? - from Ron in Alaska

Wow, thanks Ron for the fun question! Ron dictates that I must answer this without personal testing. Well then.
I'm sure you can imagine the following scenario: Father: "Son, I found this pot in your room. This is dangerous! How much are you smoking?!"  Son: "Dad, I just tried it twice! Everybody tries it! I bet you did when you were young! Bill Clinton smoked pot too..."  Father: "It was different then! Everyone did it, and pot was much weaker and safer back then." Nice try, Dad. How do you know pot was different "back then"?  Did you save some from the 1970's, take your son's "modern" pot, and take it to a lab and have it checked for potency? 
The effect of marijuana/pot/weed/cannabis comes from the active ingredient, tetrahydrocannabinol or THC. While pot growers have gotten better at producing high-THC plants (more profit per plant), pot growers have always grown different varieties under different conditions to provide "gourmet" types of pot, etc. Also, the local market, your local pot dealer, the age of the pot, method of storage, method of ingestion, etc. has a lot more to do with the specific potency of any one joint or baggie. Pot is pot, always has been and always will be.

Now for medical marijuana. 15 states and Washington, D.C. have legalized medical marijuana, including where I live, Colorado. However, the FDA has not approved marijuana as a therapeutic drug, and marijuana is currently under the jurisdiction of the DEA (Drug Enforcement Agency). The DEA requires that the FDA be the only source of medical marijuana for medical research purposes, and as such, there really have not been any good scientific studies on the use of medical marijuana. The FDA has stuck its head in the sand on this one! In 2006, the FDA released a report on medical marijuana and concluded that it had no real benefits - but - they hadn't really tested anything according to good science. Because the FDA has not approved marijuana for therapeutic use, it does not regulate the production, sale, or distribution of medical marijuana.
Pot has been reported to help patients with pain management, improve glaucoma, stimulate appetite in patients undergoing chemotherapy, reduce anxiety, treat mood disorders, and even improve the behavior of children with autism. As I've stated, most of this evidence is non-factual and the real research has not been done. However, these general effects of pot have been well-documented and the evidence stacks up in favor of pot for the treatment of some conditions. The jury is out until some real studies are done.
Since the FDA does not regulate pot production, sale, or distribution, each legal pot dispensary is essentially regulated only at the state level. The medical pot growers are unregulated as well. State regulation is patchy and still under development, so if you go to a legal pot dispensary, there is little guarantee of the potency, purity, safety, etc. of what you are buying. As the Offspring would say, "Oh f**k, I got a baggie of oregano!" Pot heads, I mean, medical marijuana users, will find a dispensary they trust and stick with it.
The social consequences of legalized pot are still playing out.  Since Colorado legalized medical marijuana, dispensaries have popped up literally everywhere. Cities are scrambling to enact ordinances to control the location and availability of these dispensaries. In Colorado, some "colleges" and "universities" have popped up that cover coursework in medical marijuana production, harvest, dispensing, regulation, etc. To my knowledge, none of these are accredited schools with respected programs. This is a case of regulation lagging behind reality, and hopefully, if medical marijuana sticks around, the FDA will step up with nation-wide jurisdiction to ensure a safe and reliable supply.
I will not go into the arguments for or against pot as a "gateway drug" or "harmless." Those are questions society must answer with well-designed, open-minded studies.