There are so many improvements on the N900 since its inception that I think it would be difficult to list them all. But it's dumb to start listing them just before PR1.2 is released. Of course I'm assuming that you want to include all the changes the community has brought about, a huge number.

Edit: To be clear, every program written or enhanced for the N900 counts as an improvement. It's not that unusual for improvements for be made on individual programs practically every day (or more). Look at fMMS or Witter or the overclocking thread for examples. Try listing all of them, even.

You might want to forget software enhancements and just count something else. That would be completely blind to what is going on.