I guess it depends on what you want from a development board.
On the one side, you have the Arduino, which is hardly more than a processor with voltage regulator and USB-communication chip. For this board, you have to externally attach everything you need. The good thing is that it is small and cheap (fortunately, since it contains that little), you can also build it permanently in your applications.
At the other side, you have boards like EasyPICx, which contains (or has space reserved for) a lot of IO onboard: lots of buttons, switches, leds, room for LCD-displays, programmer, some sensors... However, because all of this, it doesn't come cheap, and you can't really call it compact. So it is basically meant for experimenting and developing (complex) projects.
And then there are a lot of boards between these two extremes, which fill the gap. You have e.g. development boards from
olimex, or the board from **broken link removed**. The advantage from the latter is that it is still compact, but that it already contains usefull IO, such as leds, buttons, LCD display, motordriver, USB communication, ... If you want to build for example a robot, you don't need much additional stuff with such a board, and its compactness makes it more "mobile".
So, it depends on what kind of developer/hobbyist you are: if you want to make most of the hardware yourself, or if your focus is more on developing and testing programming code, or if you just want to get quickly and easy started with real projects.
Another important aspect I think is if there is good support for the board (documentation, schematics, forum), and for beginners that there are some good tutorials available. This is the case for some popular boards (Arduino, EasyPIC, Dwengo) but many boards don't offer this level of support.