For me, if it's a position player, it's someone who "has business being in a lineup." Someone who can play a position every game, or be a very good platoon player. For pitchers, he's either a better than average reliever, or if he's a starter, he wouldn't be confused for a long man. It's subjective, obviously. When someone asks for a "decent prospect" they're looking for someone who isn't going to end up being too much worse than the guy they're trading away, who is, at worst, "decent."