"Concern #1 second hand smoke. A heavy marijuana smoker could do some serious damage to others in the household, especially kids, whereas alcohol must be ingested. You can't tell me all/most pot users are responsible enough to send the kids away while they smoke their precious pot. "
Indoor air pollution is higher than outdoor levels even without smoke from smokers- if you take steps to improve the technology to handle indoor pollution this would also solve 2nd hand smoke.

"Concern #2 Driving while under the influence. We have limits to how much alcohol can be present in the blood to drive a vehicle. Do you wish to have the same system for marijuana? How much marijuana makes one unfit to drive?"

You can test drunk driver by having them walk straight line, etc; you could do similar tests plus check pupil dilation. You could have more extensive drug tests which would probably be more accurate than trying to detect for alcohol- alcohol doesn't stay in system long and can naturally occur in body in small quantities

"Concern #3 I know everybody hates Slippery slope comments, but what about Methamphetamines? Cocaine? Heroine? Should they be legalized? What's the difference between those drugs and marijuana."

With alcohol in some states there is a difference between wine & beer and hard liquor. Harder drugs could be harder to get, require prescription and/or perhaps even require a "special facility" in case someone has "bad trip, etc.

I don't their should be Federal laws preventing individuals from putting anything in their bodies- other than requireing the individual's consent [meaning, they would need to be informed of possible consequences and be old enough to make mature decisions.
Any impairment of driving should be illegal (including not getting enough sleep).