The thing with wandering away is you defeat the multiplayer aspect of the server (also the spawn in direct sunlight thing was removed). Brentai is talking about some of the stuff I'm thinking. Right now Creepers are annoying and just break peoples stuff because OOPS LOL CREEPER, but if the monsters actually tried to barge into your homes, you'd have a situation on your hand and would actually have to worry about repairs and secure areas. I think thats the type of thing guys like Const want too -- to have waves of zombie clash against his fortress where he must then pick them off with bows and swords with his friends. I want that in Single Player too (I don't make vanity structures in Single Player so I don't mind if things get smashed :D). Right now being safe is pretty easy, even alone in single player. I can easily start a new map and get to diamonds within an hour now that I know whats going on in the game. There needs to be some be a sorta... 'resource sink'.
I trust Notch is fully aware of these needs based on stuff he's said, but I wouldn't expect anything in the immediate pipeline to make SMP challenging, outside of Hell for a fleeting moment.