Ok..as a captain of a 100's strong guild and a team lead in RL also, here is my insight, for what it's worth:

This article is stating the obvious and rather superficial. I like the comments below being just "/agree". I mean, what else would you say? Duh? But the people writing the email need to hear it it seems.

You are a member of a team. But remember that this means many things to many people and certainly CHANGES based on what the team dynamic is. What is acceptable, how decisions are arrived at etc.
What is common to MOST teams is that you are welcome to voice opinion. The weight of your opinion is down to how well you communcate, how good your idea is and of course your standing in the group. Decisions are made by the leaders and things move on. I am not aware of any "democracy" guilds out there personally. No guild I know lets all members vote on everything all the time. I am sure they exist as outliers somewhere.

There are two types of issue that can result in the sort of feeling/email described in the article and they are vastly different.
1) The person wants (or feels entitled) to be leader and thinks their opinons are worth more than everyone else's - including the captains. Welcome to generation Y folks. Self obsessed. Overblown sense of entitlement. "What are YOU going to do for ME now I have joined your guild" mentality.
Answer: Private conversation laying out plain and simple what the rules and expectations are. This should have been given to ALL members when they joined anyway. Make sure you repeat important points in a slightly different way because they most likely were not listening the first time. Show them the door and tell them that while you "understand their position" that they are more than welcome to use said doorway if they simply cannot stomach it. (and don't let it hit you on the way out...)
2) There really is no feedback allowed and the captains are just out of touch. The first part is that check yourself for the danger of 1). YOU, as a member, are NOT entitled to run the guild. In fact, if the guild and/or its leaders choose to ignore every single thing you say (for whatever reason) then that is actually ok.
But if there really IS a problem you need to talk to people and get a consensus. Voice the issue in a MATURE and NON-AGGRESSIVE way. Provide an easy out for the officers so they don't lose face. Try not to make them combative if you can help it. Come armed with a GOOD suggestion that lots of people agree on rather than an acusation.
If all else fails...use the door.

@DavidC

Just because your guild is the epitome of draconian, doesn't mean that every guild is. Many guilds generally like social contract theory to a degree. For example the guild I'm part of is a guild composed of an initial group of friends, and then people joined into the guild. The people who are officers are generally people who are best at their class or raid leaders. Yes, if a guild is drastically different than you are you should probably, but most guilds tolerate if not invite input because they generally realize they make mistakes and don't want their leadership to become a pit of corruption.