That is an awfully broad question. There are quite a few ARM based development boards, so there's no way to list all the differences between the Edison and all of them.
IMO, the biggest differentiator of the Edison is that it is not a development board, it is a SOM (system on a module), and a very small one at that. If you're looking for something that requires a very small compute module that will run a full Linux distribution, and that has full wireless capability, there are not very many options. The other options tend to be either significantly larger and/or significantly more expensive.
Yes, that’s a very general question. The differences may vary depending on which ARM platform you choose. Some things might be easier to do on the Edison and might work better; some might need some extra work to get them going. It depends on what you are trying to do. Take a look at the product brief http://download.intel.com/support/edison/sb/edison_pb_331179001.pdf for some additional information on the Edison’s’ specs.
I think you need to compare Edison with Raspberry Pi or BeagleBoneBlack and other boards where you can get hardware + software (embedded Linux distribution). Comparison only hardware can be very tricky, because if you do not have software support(drivers) you will spend a lot of time on them with nothing to show. You need to know what are you want and then you choose the platform.
The reason why I ( actually it was team decision) use Edison is board size and Wifi and Bluetooth integrated. Also Yocto Linux seemed very promising, in hindsight if we would knew all the problems we would have to face we would go with Raspberry PI compute module. The reason is the community for Raspberry Pi have is much bigger then the one Edison and Galileo have. The bigger the community more chance is that someone already solved your problem and all you need to do is adapt it in your design. But I think Intel tries very hard to make this community work and I respect that.