I can discriminate against smokers by saying "no smokers allowed".

With rent subject to change... what if you start the "non-smoker" out at a lower rent rate and they turn out to be a "smoker" or take up smoking later?

We don't allow pets, smokers or more than 4 people in the house. 

If you're not doing anything on May 14th, we'll be having a meetup for BiggerPockets members in Cary at Rally Point Sport Grill from 6-9.

Grilling, noise, plumbing, ...

Pot smokers always say they aren't smokers.  

that is a preference thing, personally I dont mind, but we dont have carpet in our properties, well 1 of 8 does, we charge a little higher rent to off set the pets, yea they can do damage, but so can dogs, and smokers, and dirty people, its all part of the game, you will open up a huge number of potential renters if you allow animals. just my opinion

No smokers for sure (can I put that in the lease though?)

But, you can just tell them they can have an outside grill, but no fire pit.  

We just went through renovating a cigarette smoker cat spray house (an inherited tenant) and after 3 full scrubbings with TSP, soaking slab and subfloors in vinegar several times over several weeks, then painting subfloors with Kilz, duct cleaning, etc., it finally seems to be resolved.