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.
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.