This is an issue I've seen come up more than once and for very specific reasons, lead to much frustration in the eventual culmination of "The Ultimate Raiding Guild".

What makes a guild go from a casual Heroic/Kara/ZA guild, to a dedicated, focused 25 man progression guild? One thing.

A group of like minded individuals with the same goals in mind. This fact holds true to anything that involves more than one person. In WoW however, what you're dealing with...