We haven't seen all of the other 578 billion, so we can't honestly claim that Really Simple Lists is completely unique. It could be, of course, but we spend our time making RSL as useful and effective as possible, not figuring out how to be different.
That said, with assigned items, shared lists, and activity logging, RSL is more focussed on collaborative working than most of the other "to do" style apps that we've seen. It's also been built to be very user friendly and intuitive, so you'll pick it up quickly.
We won't. The free version of Really Simple Lists will always be free.
Although we plan to introduce a premium version at some stage, the free version that you sign up for now will always be entirely free for you to use. See pricing.
Advertising would add clutter and distraction to your experience with Really Simple Lists, which are exactly the kinds of things that we want to help you avoid.