Then read this [URL="http://www.3plusplus.net/2012/02/3-con-statistics-deeper-look-at-dark.html"]article[/URL] from 3++. Some of the language could have been improved, unless you're familiar with statistics you might get a bit lost, but Kirby makes a very good point about Grey Knights. And while he only uses the results of a single tournament, from what I've seen his results stand up pretty well in a lot of the other tournaments I've been too.
Basically, Grey Knights are the noob-stomping army. They are not actually overpowered in any discernible way, and in fact may be a bit weaker than the other top tier armies, but are really good at killing players who don't know what they're doing.
To summarize the article, they looked at GKs and DE, and how the winning and losing players fared against opponents who had been winning or losing. They found that DE's performance was based on player skill rather than opponent. A DE player who frequently won, won regardless of whom he was facing. If the DE player lost frequently, he lost to other losers as much as he lost to winners.
GKs didn't show this behavior. GKs won an abnormally large number of games against opponents who lost frequently, but did poorly against players who won frequently. The success of the GKs was based heavily on the competence of the opponent. Poor players lost to GKs, while skilled players had very good odds. While on paper GKs seem OP, there are subtle weaknesses that a competent player can take advantage of to win even with an army that seems outmatched by the GKs.
This isn't to say that GKs are a poor army. They still show up pretty frequently in the top ranks a tournaments. This does provide a compelling argument that GKs are not broken, cheesy, or unbeatable. So if you're in a position to complain about how you think GKs are broken, cheesy and unbeatable, it's almost certainly because you're a poor player and not the other way around.
Or the GK player you face happens to be the rock to your scissors. Some of the less competitive armies just can't stand up to, say, Driagowing effectively. Then again, playing handi-capped isn't the best way to win, so if you are playing a weaker army list you need to keep that in mind.
On another note, the exact opposite case is true with Orks. Everyone seems to think that orks suck, yet I've seen 2-4 ork players in the top 10 in literally every single major tournament I've looked up the results for. Orks may look weak on paper, but there's something about them that makes them a top tier army, no matter what the internet says.