People criticizing no XP for killing are retards, BTW.
There are two approaches to rewarding player with character growth - goal focused and process focused.
Goal focused works best with quest heavy games with tighter, leaner world and usually involved storytelling. In such game you can assume that the main activity of player will be completing quests and achieving set goals. This approach neglects the process, in fact it doesn't track it at all, it just assumes that if the player achieved the set goal, then player has tackled the challenges barring his way somehow (regardless if it's by killing them, or sneaking, past, or diplomacy or whatever) and should be awarded generic XP (since the system has no clue how player has achieved their goal). It's simple and efficient as long as quests are the meat of the game.
Awarding additional XP for stuff like kills, lockpicking or anything else in such system is shit design, because it rewards some approaches doubly, some (creative lateral thinking using mechanics rather than pre-scripted solutions) only once (creativity is bad for you, stick to swinging your way through), while convoluted "pathological" "solutions" are rewarded great multitude of times - the more the less linear the game in question is in terms of quest solutions.
Such systems work poorly in expansive, open-ended games focused on exploration or otherwise allowing player to define their goals.
Process focused is best exemplified by use based and it works best when any goals game logic may know about are at best sparse. Replicating it with XP system, by adding XP for combat, lockpicking, diplomacy, etc. does a perfect job copying its problems, but fails to duplicate any of its advantages, like being able to track what fucking skills is your character developing. It's much heavier and more involved system, and it might be spurious for lean, goal-driven games.
Whatever your approach is, combat XP is a humongous, kludge and giant clusterfuck of derp. The only legitimate uses I can see for it are H&S games (twitchy genocide simulators) and traditional crawlers (tactical genocide simulators) due to their extremely narrow focus (killing stuff followed by killing more stuff).
If you ask yourself "what would mondblut do?" and come up with some retarded scheme involving saving and killing the same party, possibly repeatedly, or whatever, to maximize in-game gains, then the system is badly designed and DOESN'T. FUCKING. WORK.
Besides, just see what effect on Deus Ex gameplay had addition of XP for hacking and enemy kills/KOs in Human Revolution. It resulted in player grinding hacking minigame on terminals they knew passwords for and compulsively knocking out all the enemies stealthily (in pairs if possible) using awesome button to maximize XP, when ghosting by would be both sufficient and more satisfactory solution in terms of gameplay.
Where is your combat XP god now?
