Leafnote does something that Treepad cannot do: italics, bold and underline. This has to do with the fact that rtf is much better integrated into OSX than it is into Windows. But apart from that, all that separates Leafnote from Treepad Lite is that Leafnote has a more modern look. Treepad's surface has not really changed any since 1995. But "under the hood," there is a lot more hidden (which you can ignore or use, as you like). All things said and done, one may say that Leafnote is just a more simple Treepad for OSX.
Treepad Lite has much more refined search functions and even has the ability to link to other files via textual links. Come to think of it, the latter would be a great addition to Leafnote.