Arduino: Yes! You can!
It may be too short for some of the things, but for moto 2-3 words is perfect size. It is remmembered and pronounced easy. Plus if you start describing something to someone that isn't much into it already, you can lose his interest very fast. Generalized messages like that are more prone to engage a person to delve into the matter. Even if someone have sceptical opinion.
For example:
- What, do you want to tell me, you can build a robot with it?
Yes! You can!
- How about a plane or hellicopter?
Yes! You can!
- Hmm, how about to make my car or house talk?
Yes! You can!
- Sounds intriguing. I'll look into it... and thats my floor.
If we need to sell the site to more people, then we need to sell it. Not to desribe it.
The above example can be changed a little bit for questions about the site itself.
- Can I ask there how to do a awesome X?
Yes! You can!
It could be with different tone:
Arduino: For all of us!
Certanly at any case, it shouldn't be dry narrative.
EDIT:
I propose something like those or combination of them:
All things Arduino! Hardware! Software! People!
All things Arduino! Ideas! Hardware! Software!
or variation like that:
Everything Arduino! Hardware! Software! People!
Everything Arduino! Ideas! Hardware! Software!
I believe that it is descriptive enough, as it basically is summary of the site and still simple and short to be easy remembered and to stuck in the head of people. If we try to be specific, we always will omit something important or we will make it too long. Also on the picture linked by Annonomus Person, most sites have description about one row, so I believe it will fit there nicely.
P.S. If you like it, or you agree with the general idea, please do post your own opinion or your own sugestions and/or variants. More people participating, the better.