OK let ME write a little scenario for YOU :
"Let me set up a scenario which illustrates my point. A DM runs a campaign for two groups of players of with characters of equal level. On different game days, each group is tasked with exploring a crypt guarded by mummies in order to find an entrance to a dungeon below it. Group A makes thoughtful preparations before entering the crypt and explores it cautiously. By scouting ahead, group A avoids an ambush set by the mummies and is thus able to battle them on more favorable terms. By uses sound combat tactics and thoughtful use of their spells and abilities, group A defeats the mummies handily, receiving only minor wounds. The next day, group B simply marches into the crypt without a plan or clue and are promptly ambushed by the mummies. They fight poorly and fail to use their spells and abilities effectively. As a result they are badly mauled but still manage to kill all the mummies. When the two groups next meet, the DM awards xp as prescribed by the D&D rules. Each group killed five mummies so each group gets exactly the same xp.
Only the group was followed in by an NPC party who now ambush the party on the way out to steal the minor atrifact they recovered. Party A ever professional spot the ambushers in time and being almost at full strength blow off the thieves who expected much weaker opposition. Party B died like dogs."
I see two problems with your arguments about leveling, well no 3:
1) Your not complaining about leveling your complaining about EXP awards.
2) The DMG specifically states EXP awards are for DEFEATING a situation, not for killing/disarming.
3) The DMG already covers Ad-Hoc EXP awards, but by the definition of being Ad-Hoc its clearly going to be a much fuzzier system.
Adressing number 2)
I have been through several long sessions where not a single dice was rolled, as it happened a character had be ploded to various planes and we had to negotiate a true ressurection for him. We got a very large EXP award for this, we defeated the problem, through inginuity and roleplaying.
That said a solution is a solution. If I choose to get around the pit trap by laying a plank across it how is that any better or worse than flying over it or disarming it ? If I get past the gaurd by killing him under silence how is that better or worse than assassinating him when he sleeps ? Good plans, like I showed above, are rewards in themselves, You have a greater chance to survive, you use less resources and generally do a better job. Sure party B might have got away with the zombies, but in the end their blundering got them killed. Parties who play badly dont need awarding less EXP, they will just die more if you are setting them appropriate challenges.
As It happens our group pretty much always now involves campeigns where all combats are long protracted and well abouve our AGL because we are veryvery strong tactically and if the DM didnt throw bigger mobs at us there would be no challenge. Conversely if we dont play well we die alot
As for non combat EXP, well that is clearly exp combat only groups arent going to earn. It is the DM's responsibility to make time well spent roleplaying as rewarding as time well spent dungeoning. If your DM cant cope with awarding EXP in this way its his fault and not the fault of the system