Top features in a web based project management tool?

Project management is a huuuuge term that can cover a lot of different apps. I'd say, "specificity". There are already more online project management apps that you can shake a stick at, but if you can make one specialised for a really specific kind of project (e.g. a specific industry - NOT web design), you might be able to do something better than what's out there already.

Beware of building a generic PM app. It's a very crowded field, and unless you have a really, really good insight, and the chops to market it well enough so that you get some market share before everyone copies you, it won't be worth your while.