This is a very crowded space, but one that still has a ton of room for innovation.
I would gladly pay a hefty ransom for a seamless workflow between my MacBook Pro, Android device, and iPad. Sure, now you're thinking "there's a mobile site, and the apps are coming soon!" but coming soon still means free. I'd pay $20-30/month for this workflow, plus a fee for each app even.
Currently I use Remember the Milk (Pro Version) because I love the Android client. In fact, I never use the web client. I really wish there was a native desktop client, but current attempts leave a lot to be desired.
Personally, 95% (actually, 100% but if I had a good desktop client it would be 95%) of my task-related entries are done on my phone. I may be an outlier but it seems that the approach shouldn't be a web app, but an array of mobile apps.
I've tried Wunderlist, but the Android app is very buggy and slow.
For some actual feedback on the product, it has a great design, looks clean and usable.