Loot as XP: perfectly fine abstraction and very useful especially in pickup play but it means you can end up with whole sessions at a time where either nothing is doled out or loot must be contrived in order to maintain the abstraction. Has its own in-universe problems with a lot of inelegant solutions like money eating bugs and Adventurers' Guild dues. Quick drive by on adventure guilds: fuck dem.
Monsters as XP: great and all but if your party gets really interested in conquering the kingdom through rumor and subterfuge not much XP to dole out. Also I think this is where the legend of the balanced encounter got born. The only way to level apace is to always seek out yellow or red enemies (to borrow City of Heroes metrics) which always has you jusssst surviving through brute force rather than tactic, or just as often dying to purest bullshit.
Both of these are better when used together.
Session-based advancement: ok so this is either secretly ATTENDANCE based advancement or CALENDAR based advancement. Either you enforce this for player characters showing up for a session in which case those of us with erratic schedules are left in the dust....or else you just let everyone level up every time you DM four sessions or whatever, which from my side of the desk in Encounters always seemed to mean people just showing up whenever. That's fine for Encounters, that's its purpose, but I want people a little more excited to show up for something I've worked hard on.
Challenge as XP: skill challenges, really, but non-skill-system games still have a lot of this with XP for traps and saves.This is always in concert with something, usually a couple of things, under the XP For Everyone mentality. To me these really seem impossible to balance. Either the thief and the cleric getting the same XP for the same balance beam routine is on a wrecked curve or each character has their own XP yield for a task depending on relative difficulty? Or my favorite kind of skill challenge, use whatever or these five skills you're best at. Zzzaag.
Milestone-based advancement: the only way I see this working out great is if your notes and/or adventures are very linear. Halfway through your notes, level up, you solve the dragon puzzle altogether, level again. Otherwise this seems like actually a LOT of tracks and I speak from experience: it doesn't go well. If you get that to work you probably still have a party leveling up at wildly different paces. I'm fine with that, it's the Basic Red way. A lot of groups aren't.
A long, long time ago I put an abstraction into my game that I felt was a sensible artificiality: you can only level up after you sleep a night in a bed. Not a bedroll, not some straw, not a sylvan glen. A bed. If you survive all the way back to civilization and relative safety and actually attend to your character's needs THEN you get that level you earned. This helped in a lot of ways, just one of which was to immediately surround characters fresh off an adventure with more plot hooks and ways to liberate them from their hard earned gains.
I am pondering a new, somewhat similar abstraction in an attempt to sort of combine all of the above advancement methods into one stew:
Dungeons = XP
Less tracking of the individual elements found within a dungeon that generate XP. Same end result. At lower levels you need to complete a number of dungeons equal to the level you want to attain. Say, until you reach Name Level. After that you don't have to complete 16 dungeons to advance from 15 to 16. You can make it up of dungeons of lesser levels.
What? Thanks to Wonder and Wickedness and Vaginas are Magic the new hotness if ditching spell levels. Spells having levels was dumb in-universe, too. If we're ditching one dumb thing why not just shift it over to dungeons? You know, the major part of the game. If you use Clerics then let this be a free power for Clerics to have: insight from their gods on the precipice of a sepulcher, wisdom which they pass along to their comrades, which translates into the DM saying 'Hey guys this is a level 12 dungeon.'
You survive to the end of a dungeon, you level up. You die halfway through or join the dungeon in progress, too bad so sad, you do not level up. Your reward is just the coin, the gear, and the fun of playing D&D.
This probably isn't the exclusive form of XP in the world. I still like the idea of exploration-and-lore-based "xbox achievements" hidden throughout the world like treasure for players to hunt. You do some politicking, romancing, businessing, something worth a little sumpin-sumpin, sure: have some XP. You can s l o w l y accrue enough of that over time that you and your friends can pool your XP resources and chip in to buy one of your party a level or some such. You can, in fact, as it has always been, spend 99% of the game doing whatever the hell you want. Let your down time become your full time and spend the damn campaign blacksmithing like a bad Fable game (you know, like a Fable game) or researching new spells to turn the invading army's weapons to 9HD monsters in their hands.
However, if you decide the duke's assassins are enough of a threat that you need better HP and saves, if you want a gift that no other suitor can challenge, if you just decide you want to roll dem bones and see if you end up learning Crossbow-To-Manticore... then you have to work for it. You have to risk much to earn much. You gotta do a dungeon.
One advantage of this is how important preparing the dungeon becomes, which translates to the real world as scheduling the important level-up dungeon well in advance to give anybody who wants to hit it up ample opportunity to juke their shhhhedyool. I run with a lot of people whose reaction to doing something like a traditional dungeon is lukewarm. Looking forward to that/being nervous of that is something I'd love to recover by any means necessary.
Why is this how it works? I don't know yet, probably wizards or gods but I haven't figured that out. It's not like you glow with video game ding radiance once Acererak falls down: a terrific experience affords one terrific experience. That's a 1-1 conceptually and linguistically.
Super obviously yet to be able to implement this. Soon, tho.