I am totally speculating. I was just on their web page trying to find some answers to those questions, because I immediately had them too.
Here is what I can gather, with no indication (that I can find) of the real answers to my questions, but here it goes...
Question 1) What makes this compatible with IOS devices vs their other products?
Speculated Answer 1) Their other products use a USB port to connect. Their other products replace the built-in wifi radios. One of their existing products amplifies incoming and outgoing signal and the other one is built for speed. But both are USB devices. The Edge is not. It's a standalone device. Additionally, I am guessing they are going to release software to manage it via an IOS device, as well as for the Mac.
Edit: I'm not sure about the management software. The article says it can be managed without installing software. So maybe it's a web page or something when you connect to it.
Question 2) Why Macs and IOS devices? Why not PC compatible as well?
Speculated Answer 2) I'm guessing it will be compatible with PC devices as well, but the management, at least initially, will probably be for the Mac and IOS devices.
Edit: Good question. I would think it should work with a PC if there's nothing special about software. Hopefully it does.
Question 3) Why is this device so special? What does it do that an Airport Express doesn't do already?
Speculated Answer 3) Again, I'm guessing that this will function like two Airport Expresses connected together via a cable, with one acting as a wireless client, and the other as an access point, re-broadcasting the connection to local wireless clients. You can set up one of their existing products like this, but it requires the local radio in a computer to rebroadcast the network for local wireless devices.
I have personally set up two airport expresses like this. If I could get it in one device and managed easily, that would awesome.
I just want to re-iterate... my answers are TOTAL SPECULATION with no facts to back them up. It would be cool to get confirmation but the info available is pretty limited.
Hopefully that helps.
-Tom
Edit: It looks like GeekLawyer backed up one of my guesses. Sorry GeekLawyer. I was typing my message so I didn't see yours until after I posted it.