Blaine, I really like this simple cash system. I've been researching the various ones out there and found that ucash is no longer supported, and vbux is way too feature-rich for what I want. So this is a nice simple system to start.
I think you asked for suggestions, did you not? Well, here goes...

...I want to use the cash system to encourage my members to participate on the forum and you have started off with some good things, like rewarding for posts, or starting threads. But how about giving points for things like....
- creating a quiz (Inferno Quiz, which I have not yet installed)
- taking a quiz
- using the chatroom (basically, just simply clicking on the chat link would do it)
- reading a thread (yes even lurkers need some reward! LOL!)
- thanking a post (I have the Thank You hack installed, so if someone clicked the thank you link it would be nice if they got points for it)
Some other things to mention...
I notice that I have to make a choice in the points system. One, I can give points for the number of characters a member enters in a post OR I can give points for new threads and replies. Is there some way to use BOTH? That is, I want to give members points for making a reply, but I'd like to give additional points for the LENGTH of that reply (as it stands now, a poster that posts 2 words gets the same amount of points as someone who posts several paragraphs). Also, the points are out of skew for the number of characters...1 point for each character in a post? That's too much. Could that be altered to something like 1 point for every 20 words?
Finally, in the admin CP when I manually give out points...would be nice if there was a text field "why" I was giving points and then if a PM could automatically be sent to the member that I had given points to (ie. "Hello LongtimeMember, I have given you 25 points for your helpful reply to Newbie on how to add an avatar").
Another thing...will you be creating an iShop to use with your iCash? Not essential as I think I will just create a thread that lists all the items people can buy with their points and then sell them manually, but it would be cool if it was automated and customizable (ie. only the things I want for sale in there).
Thanks again for your hack, and I hope my suggestions are of some use.