Aethelstan said:
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. Onbattle 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 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 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.
Is this fair to group A? They played the game far more skillfully than the slackers of group B, yet still get the same xp. What do both groups learn for this? Just muddle through and kill things, you’ll level up just as fast regardless. [...]
Okay, define "thoughtful preparations". Do both groups know about the mummies? How? Did one ask the local population about them? Did they use gather information and knowledge: religion to discern their foes and thus prepare for thier weaknesses? What if group B didn't have necessary ranks in those skills? What if the PCs botch thier skill roles and don't learn about the mummies in time?
What if group B lacks a rogue or suitable sneaky character? What if he fails his sneak roles? What if the cleric prepare Find Traps and not Detect Undead? What if they lack a cleric to turn the mummies or heal the mummy rot? What if the fighter rolls like poop and the mummy crits him twice? What if the wizard casts all his offensive spells to cover the fighter's retreat and then runs out of spells and ends up nose to rotting-nose with a mummy? What if he memorized charm person (for interogation) instead of magic missile? What if the rogue goes to set up flank and gets dropped?
What if party B consists of a ranger (FE: orcs), an enchanter, a druid and a bard, while party A is a fighter (tank), evoker, cleric (sun and good), and rogue?
What if the reverse was true? Party A's sound tactics fall to poor rolls and the mummies get lucky criticals, while party B takes them out with a direct approach hack and slash with heavy artilery spells and wins in one or two rounds?
The D&D XP system is designed to cover the essential randomness of combat by rewarding everyone mostly equally. Even the best laid plans fail, and the worst plans have remarkable success. The reverse ALSO holds true, but entering every dungeon/situation like SWAT members requires a certain type of player, group and mindset. Don't hate D&D's XP system for the fact your players are better lucky than good.
Lastly, set up encounters that require brainpower, such a traps, puzzles, hostage scenarios, difficult terrain, or multiple simutaneous objectives. Reward PCs for these, not the orcs/mummies/whatever set up as distractions. A few dead hostages later, they'll wise up and change their tactics.