There are basically two philosophies of loot that determines a lot of guild attitudes, rules and restrictions. I find that you can generally trace "why" a guild has the rules it has to which attitude is more prevalent.
Loot as Reward: Under this view, loot is the reward for putting in the hard work, the carrot at the end of the line. Getting an epic weapon, or completing your set of armor is a goal in and of itself.
Loot as Investment: Under this view, loot is a means to an end. Getting better loot makes your character more able to take on new challenges.
Guilds that emphasize Loot as Investment tend to have more restrictions on loot distribution. Class restrictions, or officer-assigned loot are common hallmarks of this philosophy.
Guilds that put more weight on Loot as Reward tend towards less restrictions, and more free DKP systems.
So which view is correct? Half the problem is that both views are correct. Loot is reward and investment. Indeed, you can even argue that some pieces tend more towards one side than another (class sets versus non-class sets, for example).
However, a lot of times you get problems when two people with different underlying philosophies clash. When one class takes an item which would be "better suited" for another class.
But there are a fair number of raiders who are in it mostly for the loot, for the reward aspect. (Witness all the complaining about TBC raid loot on the forums.) Gearing up to beat newer challenges is merely a way to get better loot. Even though it's not considered polite to say that. They'll mouth words like "overcoming new challenges" to get into the guild. In my opinion, there's nothing wrong with being motivated by loot, so long as you aren't disruptive about it. It's a fact that people like to make their characters more and more powerful and can see that as an end unto itself.
Overemphasizing loot as investment demoralizes a lot of people, and can make them feel like they aren't getting enough reward.
I think that a guild who is looking at setting up rules needs to take a strong look at which philosophy it is inclined to, and set up their rules to acknowledge that. However, different rules can reward different philosophies.
For example, under Officer-loot systems, the guild is the investor who makes decisions. Under a normal DKP system, the individual player is the one who invests in their character. Both can use the loot as investment philosophy, but on different levels. But only one can really accomodate the loot as reward philosophy as well.
I also don't think that one philosophy is better than the other. Different people are motivated by both in different degrees, and ignoring human nature is a quick route to failure. How people act upon that motivation is the important part.
Edit: I'm not really happy with this post. It doesn't really convey the sense of how *important* these philosophies are. Decision points such as "Gearing up the main tank first", or whether to have an "A Team" for Karazhan, or many other of the questions facing guilds have their roots in the differences between these two philosophies.