First, let me make this very clear; wanting to win loot is not being greedy. There is a very healthy line, easy to see from a mile away, of what is healthy progression and what is just being people selfish and greedy. Expecting the best loot, or even any loot, on every raid is not realistic but to get it at least one in a few weeks is good for you. Note that I used the word "progression" because I believe there is two types of progressions in this game.
One, is the typical raid progression where you down bosses and complete raid instances within your guild. The top raiding and progression guilds of a realm are often the iconic examples of this as they compete for the first kills, or at least to be among the elite who get further than say the 90% of the rest of us.
However, my personal favorite type of progression is what you do on your own. This starts when you begin to level your first character up to level 80, and then continues as you begin to tackle the harder instances, progressing to heroics and eventually raids. The thing about being level 80 is that progression can no longer be monitored by just watching the number on your character increase from day to day. Instead, you monitor the gear you have, allowing you to maybe dps better, heal the more demanding encounters, etc.
This is where gear plays the biggest role and once you begin to hit raids like Naxxramas, it becomes one of the more prominent measures of progression, and here is the point I was trying to reach with my earlier post. It is not uncommon for people to lose heart, when time after time they lose the rolls for better loot, feeling incapable of progressing their game while others are seemingly plowing ahead. This can go as far as break guilds up as people leave, hoping to have better luck in other guilds or spiking arguments within the community.
The issue is more so, I believe in smaller casual guilds, who do not have the numbers to pull their own bigger raids, like 25-man Naxxramas for example, and have to PUG to get full groups. The worst part about this being that pugs are not only unreliable, but any attempt at controlled loot distribution goes out the window.
The important thing is that during these moments, the guild is there for the each other. If to just say "where're for you mate" or even go as far as giving a piece of loot to the guild, if you won it instead of him. After all, guilds are community and raids about team work. There is no better show of solidarity than that and does wonders for the everyone's spirit.