I've used 37signals tada lists in the last few months and the main pain points are:

1) no indenting 2) cumbersome reordering 3) editing sucks

your tasskr fixes all the above issue, well done.

for me what is missing to make the switch is sharing (anybody in the team is allowed to add or check an item).

if tada list is one of your main competitor I'd add an import page to make the switch easy (I'd prefer pasting the HTML in a textarea instead of give you my credentials).