Here's my 2 cents on YNAB and what would make it even a great tool.
1) Allow syncing via the cloud, across multiple devices, iPhone, iPad, Desktop, Laptop, etc. This way when I enter a transaction in my iPhone, it is automatically sync'd with all my other devices running YNAB. I already store my data on Dropbox so it's in the cloud now.
2) Create full-featured iPhone and iPad apps (two separate apps, not just one that runs on both) so that everything available in the desktop version can be done on the iPhone/iPad. Again, if the data is in the cloud this should be possible.
I would certainly pay more for YNAB if features like this were added.
P.S. Forget automatic downloads of data from financial institutions if that's on your radar. This is VERY low on my list of things I'd like to see in YNAB. The above two are at the top.