While I agree with @Coin (that always makes me feel bad) about there not being an one XP system so they'll all need to be adjusted to the type of game you're running, that doesn't mean we can't still take a few lessons from online games which use various methods as a way to measure progress but also as incentive for desired behaviors.
For starters we need XP to trickle in capped. If it didn't then people with waaaay more time on their hands than others would dominate utterly. Even in games where the distribution method exists but is capped high some players will file endless streams of whatever-it-takes to reach that cap; taking dramatic failures and breaking points for everything, for instance, could swiftly lead to powerhouses in nWoD 2.0.
Now, let's take games like WoW. WoW started out with no way for newbies to catch up at all, but the cap was still very tangible; you could reach a state where you had your full item set and complimentary and that was it; until more items came in, the character was 'complete'. That led to players losing interest between content updates. A secondary issue arose which quickly became critical; new people coming in couldn't play with their friends, since everyone was doing 'end game' content (say, three patches with improved items past launch) which they'd need to gear up for but they couldn't, because no one was raiding the old content so they could get outdated items from previous content updates.
This applies well to MU* if we consider the abstraction here. Consider the following:
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Oldbie characters need to be challenged, or they will get bored, so increasingly high end plot has to be present for them.
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New characters come in who need content to match their own more limited abilities. Stick them in a high-end plot and they're useless, stick oldbies in theirs and they 'solve' everything.
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If there's a total cap of XP you can reach then players reaching it get bored. Roleplaying games are built toward advancement, it's no fun when you run out of shinies.
It's not as clear cut as that of course since roleplay isn't about winning encounters like WoW is, but the point still exists that some way of compromising different levels of players should exist, especially in the ever-existent lack of enough plot-runners.
To make matters trickier not everyone enjoys playing a proverbial sidekick - if the most you can earn in a week is X XP then new players will always be lagging behind older ones, assuming roughly equal amounts of activity.
I believe a catch-up method to be the most efficient way of dealing with these issues, but safeguards need to be in place. That is, allow XP to trickle in faster for those who are behind the progression leaders - but don't allow that TR silliness where new characters came in with triple Masteries despite Awakening the day before. For example:
- Allow twice the rate of XP for characters until they reach 90% of the leaders' total gains.
- Spending delays (more liberal than the normal ones but still existent for new players).
- Background checks. If a PC is an IC fledgling they get a smaller amount of catch-up XP.
- Promote activity. Too many games allow 'backup PCs' to be set up, who basically are gotten through CG and just accumulate XP without being played. XP should be a yummy carrot towards a goal.
- Allow XP transfers in character death/legitimate retirement. There is no reason to not do this other than people doing it constantly. Much of the OMG-I-DIED! drama can be avoided with just that.
- And above all make sure your cap is set high but not too high, or some players will abuse it. Constant. Breaking! Points!
You may want to allow some things to encourage plot running besides the above but those are the basics of what I'd consider a decent system. Good luck defining 'activity' though. 