I think you are coming to the wrong place for an unbiased opinion. Most of the people here probably lean towards using it, and haven't bothered to find out the arguments against it.

From my experience through life, marijuana is bad because:
1) It destroys brain cells.
2) In addition to other brain cells, it destroys precious memories.
3) If you smoke it, which the vast majority does, than you are dramatically decreasing your life span.
4) In addition to destroying your lungs, many other vital organs are adversely affected when you smoke, including your liver and heart.
5) Marijuana slows metacognition. It makes you understand and think more slowly.
6) It slows reaction time, so no driving.
7) If you smoke it, your clothes, house and car smell like smoke, and you also have to deal with all the other annoyances of smoking (like ash and stuff).
8) If you grow it it takes time and money. If you buy it, it takes a lot of money. If you sell it, someone who you thought was your friend will rat you out.

And now the good points:
fill in the blank here.

I'll keep smoking, at least for awhile. Too many good points. Trying to cook with it and use the vaporizor more though.