In my opinion loot whores are nearly as abominable as loot ninjas. Sure -- everyone wants to be their personal best, but WoW is not a single player game. When I hear a guildie bragging about their WoW-Heroes score, I usually inform them that it means they should be working on helping others get caught up.
In a raiding guild, the object of gearing up is to help push your team in progression. If you're not interested in the greater good of your guild, you may want to reconsider being in that guild or your desire to raid at all. Yes, it's fair to want and hope for those shiny purples, but odds are good that someone is going to be miss out. Take heart! Kel'Thuzad will be there again next week.
Individual guild members have a responsibility to help push their gear score/effectiveness. The tokens system allows players to be in command of their own upgrades. Having trouble getting Tier Shoulders? For 60 emblems of valor, they're yours. Properly gemming and enchanting your gear promptly helps to push up both your gear score your performance. My main is a Skinner/Leatherworker, but I carry gems and enchants with me so that I can shift on the fly when I get a needed upgrade. Be aware of what's available to you, and keep those goals in mind.
Play the gear that you have to its maximum potential. Someone who is undergeared and still keeping up is not only admirable, but proves themselves worthy of the best gear. I think we've all seen the Shaman geared to the teeth who can't heal his way out of a paper bag and the Warlock in Level 78 crafted blues who pulls 2.5k on Naxx 10. In most cases it's not necessarily the gear that you're wearing but what you do with it.
When there's a piece that you need or want, make it known, but don't be obnoxious about it. In my guild we do a "Wishlist" where folks who have run raids over and over can name a specific piece that they feel completes their character. They will be the first to be considered for it. And keep in mind, at the end of the raid, only one person will walk away with the Betrayer.
If you're continually being passed over, there may be an issue with your guild or your loot system. Talk to your GM, and they may be able to explain their motives and goals. If you earn your keep, you should be rewarded for your deeds.
Once you are geared as well as you can be for a particular raid, you are far from done with it. Now it's your job to help others along. This brings the team up to par and move on to the next challenge. "But I don't need anything out of Naxx 25." Yes you do: geared guildies. Unless your entire guild is fully geared for the next raid, you have a duty to get back in there.
Gear envy can rear its ugly head in any progression guild. Team spirit goes a long way. No matter how great your gear, you will not be soloing Ice Crown Citadel any time soon. It is in your best interest to keep the best interest of the team in mind.
WoW.com offers a plethora of information on guild leadership and guild membership. Be sure to check out Scott Andrew's Weekly Column Officer's Quarters and keep an eye on the community with Mike Schramm's Guildwatch.