I've seen so many "collaborative task managers" -- I doubt that the tool is the problem. More the fact that people don't like to do this type of tracking, reporting, etc.
I am often tempted to make my own. I am held back by the fact that there are so many out there and I am unable to articulate what is wrong with them exactly. Which leads me to believe that I am not really looking for that kind of tool - I just think that I am.
The basic problem is that if you ask 100 different project managers what they want you'll get 100 different answers.
Every project manager (I'm using a very broad definition of project manager here...) wants a system that is almost but not quite what someone has already built. "Ohh this looks nice - wait I can't do X? Then it's totally useless." X is different for everyone. Some people can't live without a versioning system, some pople can't live without tasks that have a sign-off field, etc. There are literally hundreds of ways of doing it.
Nobody has solved this problem yet. Probably because it's extremely hard.
Most people will keep to-do lists, many people use paper lists. The reason why so few have been successful or broadly adopted is because they don't cross the email/task/project problem.
Not all task managers are built the same, but the need a solution is ubiquitous.