I like the zero-sum idea for skills.... sounds great. And you should lose exp slowly for quite a few skills.... say for weapon skills 10 - 20 mins every 10 hours online should stop that.
For power leveling and so on, there is a simple solution. No levels. Think morrowind style (where each skill has levels) but without any main character levels. Especially the part where you can only level up a skill by using it or getting a trainer. With this system there is no way a character can spend three hours crosbowing bunny rabbits and learn how to cast a fireball.
I think for weapon skills you should have not just bash / slash / stab general skills, but a variation on these for EACH weapon. So you have sword stab / bash / slash, slash and stab woul be pretty powerful for most swords but bash (hitting someone with the flat of your blade or pommel) would be pretty cruddy), for a club bash / poke / swipe, with poke doing nothing much (maybe with a chance to wind unarmoured characters you poke in the stomach though). In battle you would be able to select which strike to use, and be able to qeue up sequences - say slash, slash stab. Improving your stab skill for the sword would increase your slash and bash slightly - after all you\'re handling the same weapon, with the same balence etc. Also if you use a certain combo / sequence a fair bit you could get a \'skill\' level in it and get a bonus when you use that combo.
A bit more on the sequence bit.... say you have the interface window looking a bit like
. __ .. __...___...___.. ______
| B | | S | | T | | L | | S S T |
----..----..----..----..----------
You have your Bash, Slash an sTab buttons, then a Lock button i\'ll explain in a sec, and then a box which fills up with the next 5 or so moves you select. In this case its Slash, slash, stab qeued up. Now as it is this isn\'t a combo, just the next three attacks you will make. But if you press the lock button, it will lock those three moves into a combo, where each action will execute faster, but doing less damage. However once you practice up that combo, it will take as much time as two single moves and do almost as much damage as three moves.... Or something like that.... tweak as you like.