An ad should mention music style. For instance a reggae band or a country band or a Christian band. would require certain knowlege. A well rounded musician could play all those, but such styles might give other musicians pause because they might dislike a certain style or just not feel adequate to play it well.
I have seen ads for musicians in Christian pop or other bands state that the musicians must not smoke, use dope and must live a "Christian life-style." To me it is OK to state such restrictions if the musician's lifestyle is a major issue for the band as it would be for bands with a strict religious affiliation. Otherwise, leave such lifestyle requirements out of the ad. What do you do if the dream guitarist shows up, but he is a chain smoker? You might just decide you prefer him and his cigarettes to a so-so guitarist who never smokes.
I think it is wise to mention that you have rehearsal space if you do. That is a plus. For one thing it shows you are serious about your band and have gone beyond just the preliminary wishing and dreaming phase.
In Nashville, musician wanted ads often stated that the band had regular local gigs or was about to do a regional tour or had a recording contract or was just a jam band. The ads always stated if the music was original or mostly covers. If there was an expectation that the musician be able to rehearse a given number of nights per week, that was stated.
One other thing not mentioned in the other posts in this thread...professional equipment. Many ads in Nashville commonly state "Must have pro equipment."
I appreciate that because when I lived in South America and was in a group trying to start a band, so often we'd have musicians show up with the worst, most unprofessioanl set ups you could imagine. We could never have played a gig with them. Some even expected us to provide the equipment and transportation for them. What dreamers!
Age has not been mentioned either, but many ads in Nashville do mention it. For example, commonly ads for heavy metal bands say no one over twenty-five. Other ads will say, no one under eighteen. That is because bands that play bar gigs cannot use minors.
Another common line in ads is something to do with appearance, especially heavy metal bands. I know an excellent guitarist who was turned down by a band because he was not tattooed, had no piercings and his hair was too conservative. The band really wanted him and asked him to please change his appearance, but he refused. Their loss, but it happens. Be warned.
Lastly, many Nashville ads say something like, "Must be experienced. No amateurs." But some will say "Beginners (amateurs) welcome." That is rare, though. Oh, and I forgot. Some ads do say must read music or must know Nashville number system.
It seems as if band ads can get away with ageism, sexism and every other predjudice under the sun. If you have any limitations you wish to impose, do so. Just be aware that you may gravely limit the number of musicians who apply.