Yeah, if you can afford it, the 580s are a lot more flash for the money. The problem is for most folks $400 for a flash is a huge chunk of change, particularly if they're going
the Strobist route, and a
$130 Lumopro LP120 might do just as well for them. If you don't need eTTL, an EX speedlite can be serious overkill.
eTTL is where the camera and flash can talk together, and the camera can set the flash power based on through-the-lens (TTL) metering--the flash fires a "pre-flash" burst and the camera meters during this burst before taking the picture. This is useful in run-and-gun situations when you don't have a chance to reshoot and adjust. Because the power is based on metering, it also makes bouncing easier, because you don't have to guesstimate and adjust for the added distance. You also get goodies like high-speed sync, where you can use shutter speeds faster than your max. sync speed (typically 1/200s or 1/250s).
Some folks get by just fine without eTTL, just as some folks shoot exclusively in Manual mode on their cameras. But having it is like having a choice between Manual and Aperture Priority modes. Sometimes you want to use an automatic mode.