Real fullscreen mode for Lion and Snow Leopard (yes, you can get a Lion-like fullscreen mode with 10.6.x): You can work in 'clean' full screen mode or use palettes, outline and menus... or anything in-between - just take your pick.
iCloud support... Only for those who buy Mellel at the App Store!
Auto-save: if you're using 10.7 (Lion) you can auto-save files using versions... But you can also use the old auto-save system.
Automatic Language switching: Spelling dictionary will switch as you switch languages. You can also set which spelling dictionary to use with which language, set a language to any selected piece of text, and more.
Palettes, new and improved: Palettes are now easier to manipulate, change, arrange, drag and so on - but they keep in one place, respecting muscle memory accessing palette options... And there are less of them!
Ruler and tabs: the easy way. You can now add tabs on the fly, drag them, set exact position, change tab types, enjoy a vertical rule, snap tabs, and so on.
Template options: you can now easily set the preview, description and update palettes quickly and easily (Mellel will remember where to put the template).
Re-arranged menus: we've added a View menu, and condensed all the style menus into one.
Easy access to styles: Access the page, paragraph, list, character and section styles easily via the palettes - each style palette offers all the options, and a style list.
Document and fullscreen background: More in the fun-and-games department, but useful to those who wish to control those.