Inspired by this question, what would the effect of setting your exp to zero after levelling be? Obviously, each level would take longer. Would there be any other effects?
Note: As BESW points out, by this I mean that the level stays the same - but you effectively need to regain the exp to reach the level you're at, then some more to hit the new level.
The big change will be in the number of encounters by level.
The recommended number of encounters/level is 10 and XP budgets support this. The gap between L1 and L2 is 1000 XP and the XP budget is 100/player. From 2-3 it's 1250 and the XP budget is 125/player.
Effectively just from L2-L3 you are forcing 8 extra encounters, nearly doubling the distance between these levels. From 3-4 it's even worse; 25 encounters are required.
Here is the number of encounters required by level for most of heroic tier:
2 10
3 18
4 25
5 31.42
6 37.5
7 40
8 43
9 47.14
This quite obviously gets silly. Spending 4x the recommended guidelines getting form level to level is a grind not many D&D adventurers would be interested in taking. The lack of advancement would soon descend into boredom as there are only so many level appropriate foes that can come up. Consider that the average D&D session seems to be about 4 hours and a mid heroic encounter can take 2 hours, and most groups meet once a week. That means that by the end of heroic tier it is taking something like half a year just to level up.
My group would quickly find this boring and would lose interest.
In addition the number of magic items and other treasure per encounter would drop very dramatically as you level. Generally a party of five receives 10 or so treasure parcels per level (that's from memory though). This would mean finding treasure just once per 4 encounters by late heroic, which seems counter to the spirit of the game.
Unless your game system does something with numerically expressed experience points, the actual value of points is irrelevant.
No, there would be no effect of zeroing XP each level in D&D 4e using the standard (as printed) XP values for levels and encounters, other than levels taking a progressively longer time to attain (the MMORPG "grind" style of decelerating progression).
D&D 4th edition has a few entirely arbitrary number systems, and XP is one of them. The values as presented in the books aren't inherently meaningful; you could change those values and change all the values of XP that monsters and quests give and come up with entirely different values and the game wouldn't really change, in and of itself. All that would change is how quickly your players go up levels, and that's already something DMs have a huge amount of control over. Consider, for example, that some DMs simply forgo XP entirely and declare their players level up at story-appropriate moments.
As others have pointed out, the XP system as written is balanced with monsters, which have XP costs, and encounters, which have XP budgets, all in the intention of having about 8-10 encounters per level. You could abandon XP and just track encounters and level up characters after every 8-10 encounters.
But XP was meant to add some extra nuance or reward for characters that find creative ways to bypass encounters. If you don't want players to feel penalized for negotiating with orcs rather than slaying them, you might give the same amount of XP for the skill challenge as you would for the battle. The point of experience points, really, is so that, each time you deliver them to players, players know they did what they were meant to do.
In reality, it's just a pretty raw and basic reward point system, a pat on the head, a Pavlovian bell, as it were, to get the players salivating.
Zeroing them for each level might have some psychological effect beyond having to change the math on how much you need per level. Note that many MMORPGs zero XP each level. Also note that many MMORPGs are designed to slow down leveling as players progress. However, the psychology of zeroing is meant to make it easier for players to know how far, percentage wise, they are in leveling up: 1,247,285 of 2,500,000 is a lot more clearly 50% than 1,247,285 of 1,300,000 (when the last level was at 1,200,000, but you don't see that information anywhere).
Arguably, a simpler XP system than the one in D&D 4e would be to zero after each level and level up at a fixed value, such as 1000 XP. Then players always know exactly how far into the current level they are. Each encounter would deliver about 100 XP, give or take a few points, based on the DM's discretion on difficulty and such.
As for the effects of the raw change to D&D 4e play if you zero XP each level and make no other change, it slows level progression by about 5 times. As wax eagle points out, it creates a parabolic XP curve which rapidly slows from levels 1 to 10, going from 10 encounters up to nearly 50, and then hovers around 50 encounters per level from levels 11 through 30.
Of course, maybe you want a slow campaign with about 5 times more content per level. At which point: go for it!
