I've been to Morocco (including Marrakech), I had a great time. A lot things are common sense, like don't buy food off random stalls in the street (although there might not be so many if it's Ramadan). When buying things from souks, they will over-price on purpose, assuming that you will heavily barter the price down. You can reduce the price by over a half.
For your wife, maybe it would be advisable to wear a hat as well as covering up. It's not nice to get 'looks' from some of the men, who aren't used to seeing Westerners exposing so much skin! But round teh hotel you should be fine.
I would advise you to visit the main square in Marrakech (I can't remember the name...something-el-Fna) At night as well as at day. Beware though of people trying to sell you things, I had a (rabid?!) monkey almost thrown on me, presumably thinking I'd like it, and then they could ask for money...it's fun to watch the snake charmers though. Also the tannery is interesting, (watch out for the stench though - they give you mint leaves to hold under your nose for a reason!)
Oh and as for drug dealers, obviously, it's common sense again, stay away from them, you could be asked in toilets if you want some hashish. (I was on several occasions!) We were told before we went that they could sell you out to the police if you buy some. Cue a night in a rat-infested Moroccan prison!
As for general safety you should be fine, just keep your wits about you, and don't leave your wife alone in public places (again common sense!) I had a great time in Morocco, I hope you do too!!
Ooh another thing I just thought of - a lot of Moroccans speak French, so you might want to brush up a little (especially on your French numbers - which you will need for bartering!)