I found that if you get a good cut of meat and grille it slowly, it needs nothing but a little salt and pepper at the end after it rests for 10 minutes and is plated.
Grill a steak slowly? Never! Hot, hot grill for me. Sear the rib eye or t-bone, whatever you got, until the edges are slightly blackened. Do the other side the same way. If you preheated your grill you will have nice sear marks on both sides. Cook to medium rare. Do Not over cook!
The only marinade I've ever tried was teriyaki sauce. Place the steak in a zip lock with teriyaki for an hour or so prior to firing up your grill. It's pretty good!
Slow cooking is mostly for brisket, chicken, other larger cuts of meat, in my opinion :D
