It’s very important to allow the characters to advance in a role-playing game; building up a character is one of the major pleasures in them. It can even be quite addictive, as demonstrated by any number of MMORPG’s. Games that don’t feature character advancement in some form tend to be niche productions – and rarely do very well.
Whether or not it was a conscious design decision, classic d20 is set up so that a determined group of players can advance very quickly indeed. According to the rules, a party of adventurers are supposed to face roughly four equal-level challenges per day of adventuring. If they face fewer than that, characters with powerful limited-use abilities – such as most spellcasters – gain a big advantage. If they face more, rogues and defensive fighter-types gain an equally large advantage. Either way, after thirteen or fourteen such encounters, the party will go up a level – one level every four adventuring days. Two levels per week of game time – and one level per game session – is not out of reach if “leveling” is the groups sole focus or if the game master simply believes in keeping the characters busy.
I’ve had plenty of d20 characters reach retirement levels in possession of a selection of abilities that they’ve never even used. As a player, such advancement was far too fast to be at all satisfying – and inserting downtime between adventures didn’t really help; either the game skiped right over it (which had no effect at all from the players and game masters viewpoint), things happened but didn’t advance the characters (boring some players completely and undermining one of the major pleasures of the game), or “downtime” simply turned into an adventure in a different setting (in which case it didn’t slow things up a bit).
Stretching the timescale helps a bit. If you – say – turn all the “daily” abilities into “monthly” abilities, slow up healing and such proportionately, and turn “at will” into “once every three minutes”, you can make it three or four encounters per month without unbalancing the classes too much. Unfortunately, that makes continuous effects and charged devices enormously more valuable, makes diseases and poisons far more of a problem (can you hold on for three weeks George?), makes it hard to understand why anyone would become a spellcaster in the first place (so you can cast “Light” twice a month? Ever hear of candles?), and unbalances the setting in a lot of subtle ways. It can be interesting, but it’s awkward to set up and involves a lot of juggling to make it work.
That’s one reason why awarding character points directly, rather than in one-level twenty-four-point blocks, is one of the options in the Eclipse Web Expansion. That allows characters to improve every session and yet still have time to experiment with all their abilities. If your game awards two or three character points a session, it will still keep character advancement moving – but it will slow things down to around ten sessions per level.
Alternatively, you could use a base of one or two character points per session and award bonuses based on accomplishing goals – whether those are personal (“find my true love”) or external (“stop the attack of the evil overlord”). That does result in a different sort of game, but getting away from killing things for experience may suit a given setting better. After all, in normal d20, you won’t find Frodo sneaking past the enemy on his quest to throw the One Ring into Mount Doom; instead, he and his friends will be grinding their way through the armies of Mordor to build levels for the climactic battle with Sauron.
That will require some adjustments to the usual Treasure = Power formula, but for that you can either simply limit conventional magical items – point-buy characters can get along without them anyway – or use the wealth level templates from The Practical Enchanter.
Otherwise, in d20, it’s entirely possible for a group of adventurers to rise from obscurity to epic level, found a kingdom, see it overthrown by a demonic invasion, see another group of young adventurers rise to destroy the demon overlords in apocalyptic battle, and to see a new group of adventurers start at first level, clean out the remaining monsters, and rebuild the realm with epic magic – all in the time it takes the local peasants to plant, grow, and harvest a single crop.
I’ll admit that’s exciting – but I prefer a world with a little more stability. A world which seems unlikely to last long enough for the next generation to grow up is kind of depressing. Worse, it means that nothing the characters can ever do will actually mean anything. Either it will be quickly wiped away, or – if it was something like destroying the world – it was inevitable that some other group of adventurers would do that within a few years anyway. Since worlds are pretty obviously short-term things in this system, another one will doubtless be created quickly enough.
You can presume that the player characters are secretly exceptional, or that – for some reason – such things simply don’t happen, but falling back on game master fiat instead of logic to keep the world from falling apart is always unsatisfying. In the end, a world of rapid level advancement – and constant turmoil – is just as boring as one in which nothing ever changes. Chaos is entertaining for a bit, but watching the random swirl of colors will soon become just as dull as inspecting the fine details of a painting, however masterful.