Lately it seems like a bunch of vendors have been pushing out more dev boards in the wake of the success of the Arduino platform. Actually, this isn't quite true, there have always been dev boards out there. The boards were mostly produced by the chip makers themselves, and were fairly expensive compared to the current wave of offerings, but there's always been a steady supply of dev boards if you wanted to tinker.
The largest change has been to ease of use and a PR talking about how your board is an "Arduino killer", but does this make any sense?
Latest cab off the rack has been Microsoft's Gadgeteer platform, based on an ARM MCU and .NET development environment. I'm not picking on this board because it's from Microsoft, I honestly don't particularly care who provides the boards and the environment. I am picking on it because it misses the point. It misses it in the same way that NXP's mbed dev board and (online only!) environment misses it.
Early users only care about how quickly they can get a blinky LED, and how quickly they can interface with some prebuilt modules. All of these offerings do that about as well as each other. If that's all you'll ever do - use other people's prebuilt modules and other people's libraries - then you'll probably never run into any difficulties. There's a lot which can be achieved with that exposure.
But at some point I think it'd be useful to be able to, for some users, take those experiences and go deeper into the rabbit hole of electronics. Not just use other people's libraries and boards but make your own, dig into the internals of what's really running on these boards, how they've been designed and built, to extend those platforms into a base to explore the whole realm of embedded software and electronics.
So how deep can you go on these Arduino killer platforms? Not that deep. In the Gadgeteer case there's no PCB layouts or schematics for the mainboard. There's a spec on how they want you to use the connectors on it, and how to build co-operative modules, but there's a very clear limit placed on doing anything custom with the mainboard itself. It's hard to tell from the source code provided, but I don't see any sign of the any of the code actually running on the MCU. The same goes for NXP's mbed, the hardware driver libraries are all closed and inaccessible. (NXP's mbed does provide schematics and PCB layouts for their dev board, but only as PDF images and not in an editable form.)
Neither of these products can be a base for anything other than using the exact products they already have, exactly as they have presented them to you.
I hugely appreciate the fact that much of my own MCU and PCB work is based on looking into these details, and having access to source code running on the MCU, and the schematics and layout of the boards in a form I can edit. That in the end they're based on what the Arduino project and other true open source hardware projects provide. I really doubt I would have gotten very far with my own boards if it wasn't for that level of access that the Arduino provides.
And that's the point. It's not just about slick dev environments and an open surface to plug things into, and talking about "open hardware" like that means your have a PDF of what the pins on your board do. It's about how deep into the rabbit hole you can go if you want to. Gadgeteer and mbed will let you get up to your ankles, and no further. They'll do well and I have no doubt people will do so quite amazing things with them, but they're no Arduino killer.