[colanderman]

Eb is a weird key to play guitar in.

Van Halen did it all the time. (Tuning down a 1/2 step is not uncommon among heavier rock/heavy metal bands. I'm not entirely sure why though.)

[wildwood]

That's cool, I hadn't heard of that before. Here's a link with some possible explanations:

http://www.strat-talk.com/forum/sidewinders-bar-grille/24512...

The explanation that E-flat works better with tunings for brass and horns was interesting.

[dfan]

(Tuning down a 1/2 step is not uncommon among heavier rock/heavy metal bands. I'm not entirely sure why though.)

You answered your own question; it sounds heavier.

[Synaesthesia]

Eb in original tuned guitar is very hard. But yeah detuning it 1 semitone is a good trick.