Totally agree with the formfactor thing: Just love that you can LEGO stuff together. But even as a MsC in EE, I love that I can do this and then create a real thing from it with breadboarding, proto pcb or a custom designed PCB.
The other thing I like a lot (even more I think) is that there is such a great community with many, many examples, so you don't have to dive into the bit level to get a sensor or module working.
It's not ment to be an attack at people who use these, I still like my .Net Fez Spider for making silly things.
But. I do think that when someone is finding they want more boards, they are probably envolved enough in it to go naked, and look at the uC world.
A friend wanted some help on their project, they had never considered using the 8pin uCs available for 50p each, they where trying to tie together Ardiunos and a lot of wire. Luckily they asked for help rather than just throwing money at it and using what they know. But I fear if people just stay in a LegoLand they might not realise that there are other things often more appropriate. I do still love LegoLand mind, despite not being there for a long time.
The other thing I like a lot (even more I think) is that there is such a great community with many, many examples, so you don't have to dive into the bit level to get a sensor or module working.