Biscuitfiend says:
You might get annoying stuff like players splitting their posts into 2 posts, 3 posts, 5 posts for the extra EXP, which would stretch out the forum pages and make me sigh endlessly. Personally I prefer editing my posts to add extra detail, and it would annoy me if players posted their responses 1 sentence at a time just to farm EXP.
This ^ (just using it as an example, not singling you out Biscuitfiend) and the whole "well what if people abuse the system?!" thing all really come back to the same thing that happens when we're playing RPGs. People who min/max or munchkin are going to find ways around the system you put in place regardless of how tight you might make it. On that note there's nor real reason to try and make it so people can't break the system. You'd be better off in making the system have limits that can't be worked around.
If each post gave you 5exp for instance, and you could only gain 100exp per day from posts, then you -might- get people who do make 20 posts in a given day. But anyone who was going to post this much anyways probably isn't doing it purely for the site exp, and even if they are, that's added activity to the site and drives traffic -which is something you do want to encourage.
If you want to use exp on the site then make a list of things that you can gain exp from and how much each gives you. Then find a way people can use or spend the exp to do something, this causes a supply and demand effect for the people who really want exp and whatever it buys.
-Things you should get exp from should include: new topic, new post, new thead, new subforum, new game, new pc, new npc, new monster/mob, posting a guide/tutorial, suggesting a new idea that's awesome and gets implemented, finding a bug we didn't know about, first signing up.
These can all be varying amounts with things that can be abused being worth less exp, (new posts) while things that are useful and increase activity and community should be worth more (new topic, an npc for people to use, new monster mobs, guides).
Limit the total exp a person can gain per day or week to actually make abuse of the system nigh impossible, but to also make those who are more active obviously gain it faster than those who are on once a week. This is pretty much how you'd want to do an exp system that hinges on user activity if you're worried about people abusing something that give you nothing of any real value aside from some cool stuff on the site to add to your profile.