Reading Paladin in Citadel this morning, and the piece from ChattyDM he links to, and I had a random thought.
Haven't fully thought through all the ramifications, but here it is.
To encourage old school dungeon exploration as the default play mode (in ANY edition, including 4E) of Dungeons & Dragons, drop XP for monsters completely. Only give XP for gold. Then use the Arneson idea that characters only get that XP for gold when they fritter it away on wine, women & song, or commissioning a giant statue of themselves in the town square, or otherwise getting rid of that gold in a non-productive way.
Instantly, you've taken away the desire to be uber-combat machines (sure, leveling makes you better at combat, but it also makes you better at surviving traps and hazards of a dungeon to get to that loot). You've shown the players they need to be clever and cautious when exploring (to avoid combats and traps, and to discover all the secret hiding places where treasure lurks). Use something like my Keystone Treasure idea, and you've given the characters concrete goals to work towards while exploring a dungeon or wilderness area. Finally, using the Arneson rule for XP for gold wasted adds a level of interesting roleplay, as players get to define their characters by how they use that loot they've worked so hard to drag out of the dark places of the earth.