Normally a cleric would gain +1 to attack bonus, +1 to fort save, +1 to will save, +1 1st-level spell per day and +1d8 hit points (+CON) when he reached level 2. I've always found this mechanic kind of strange. Why, after plumbing the depths of the skeleton king's lair, does my character suddenly become so much more powerful? Shouldn't he become more powerful as he explores and fights? I think it would make more sense from an in-game point of view if he became a little bit stronger one day, learned something new the next day, became more resilient the day after that, etc., instead of all this occurring at once.
In light of this, I thought it would be more fun for players in my campaign if I distributed these gains more evenly. For example, since the cleric gains five things after getting 2000 XP, he would instead gain one thing every 400 XP:
400 XP: Attack bonus + 1
800 XP: Fort save + 1
1200 XP: Will save + 1
1600 XP: + 1 1st-level cleric spell per day
2000 XP: Hit points + 1d8 + CON
In addition, I think it would be easier for new players to grasp things this way because they aren't having to deal with so many changes occurring at the same time.
Has anyone ever tried modifying the rules this way? Are there any downsides I'm not thinking of?