I wish I remembered the details. Seriously, it was subtle enough that I've forgotten.
The Dashboard widget for recording your time was one -- Harvest's is nice, especially because you can easily switch from client project to client project with the UI. But the thing which ultimately switched me over was the ability to fully customize the line items on invoices -- I was able to get the style I wanted with a few clicks, and I can easily edit and merge line items if I want to. For me, it hits the sweet spot between doing things automatically and allowing you to edit things.
Of course, once I switched over to Harvest I gradually became subliminally aware of other reasons to like it. I haven't found myself wanting to try out anything else.
