Someone who smokes marijuana regularly can have many of the same respiratory problems as cigarette smokers. Persistent coughing, symptoms of bronchitis and more frequent chest colds are possible symptoms.
Marijuana smokers are not at risk of developing emphysema because marijuana smoke does not affect the small airways in the lungs as tobacco smoke does. Marijuana smokers do have a slightly higher risk for developing treatable lung infections. (I believe 16% greater.)

There are over 400 chemicals that have been found in marijuana smoke. Benzyprene, a known human carcinogen, is present in marijuana smoke. Regardless of the THC content, the amount of tar inhaled by marijuana smokers and the level of carbon monoxide are 3 to 5 times higher than in cigarette smoke. This is most likely due to inhaling marijuana more deeply, holding the smoke in the lungs and because marijuana smoke is unfiltered.
Coffee contains over 800 chemicals, including several carcinogens. There is no medical literature linking marijuana and cancer. Evidence suggests that lung cancer in smokers of tobacco may actually be the result of radioactive elements present in the smoke such as Polonium 210 and lead 210. Marijuana smoke is not radioactive. Filtration is not the issue, smokers of 'light/low tar' cigarettes are at the same risk of developing cancer because the filter does little to prevent the smoker from inhaling the polonium 210. Also note that marijuana smokers in general inhale much less volume of smoke compared to tobacco smokers.

Recent research shows that regular marijuana use compromises the ability to learn and to remember information by impairing the ability to focus, sustain, and shift attention. One study also found that long-term use reduces the ability to organize and integrate complex information.
No shit?
In addition, marijuana impairs short-term memory and decreases motivation to accomplish tasks, even after the high is over. In one study, even small doses impaired the ability to recall words from a list seen 20 minutes earlier.
I was never worried about pot fucking with my 'ability to learn'. My state issues a standardized test to all students bi-yearly until sophmore year as a requirement for graduation, my scores all hit the 'advanced' areas. PSAT/SAT scores also decent as far as I am concerned. Critical Reading scores better than 50% of the students the test was administered to, if you can read critically you can 'learn' just fine. I would like to see just how much marijauna 'impaired' these people and what the control factors were.