If one wanted to create a new web development tool (free or otherwise), what would it have to be able to do that would differentiate it enough from all the others to attract people's attention?
In what areas are all the other frameworks and languages falling down, such that if a new tool addressed those issues, it would stand a chance of being adopted by significant user community?
First of all personally I wouldn't go there, it's a really hard thing to accomplish unless you go Open Source and back it up with commercial stuff.
Secondly as someone in the comments mentioned you need to tell us more. Are you developing a new framework, a new language? or a framework on top of Java, .NET. Who's your target? Developers, designers, people who can't code? People who can code but want to code more efficiently? Or is it about scalability? Or ease of use? etc.
First you must 'eat your own dog food'. You must show the user that the tool can cure your pain. If you can show this, then get ready to be attracted by users.
You can create an application where the developer can select File -> New Website and everything is great and awesome, but if nobody is using it or talking about it, it will not be used.
The only way to develop something like this is doing it in a (open or closed) community form. Let developers contribute to your code, preferably by developing (since this will speed up the development) or by submitting and discussing ideas.
I don't want to sound patronizing, but you will never be able to come up with a list of features, implement them, show it to the world and then expect everybody to love it.
Your goal is not to be different, it is that people are using it. There is a difference.
So I would suggest to start there. If you can find people that are unhappy with the tool they are currently using, go for it. If you can't find them, there will be no need for a tool like yours.