I don't think the 'sleep' mechanic is there just for making sure there are no enemies around or enforcing a bit of RP. You notice how the game loads when you sleep? Well it stands to reason that when you level that loading flips the switch for the
world to level up. Remember, this is TES, and the world is level-scaled to you. So if you leveled in the wild or a dungeon (without the division of a loading screen) then- at least until you hit a load screen- you would be leveled above everything else. The enemies and loot would still be on the lower level.
Also I think they took out the 'rest/level anywhere' because waking up to a natch hound or cliff racer really svcked.
How about you add a button to the wait menu that says "Wait and Level Up" and add a button to the Sleep menu that says "Sleep and Level Up"? That way everyone is happy. Krymzom74 is happy because he can level up in a dungeon (provided there are no enemies around), everyone who doesn't want an instant level up is happy because there isn't one, Xarnac is happy because he can sleep without leveling up, and people who like the sleep level up mechanic are happy because it is still there. Everyone wins.
Something like this is best. Ideally though I'd want a way to set up a 'camp' in the wild so I could rest/level (rather than a wait/level anywhere option). But still no leveling in Dungeons. At least not if loot is still scaled to level.
So what I really want is a system where you can only level (via resting/waiting) in a 'safe' area- a city, village, house, or campsite (which you can set up). But I would be willing to go for a system with the 'Level Up option' added to the Wait and Rest menus.