Attunement

I'm a fan of it.

I thought it was good back in the days of Kara, since it showed effort had been put in to be even allowed access to the content.

And I like the way it's been semi-re-introduced (not a real word, I know) with the new ICC heroics.

If you want to be able to complete the higher level content, then I feel that you should have to work for it.

It doesn't have to be a blanket-ban on everything. But simple things such as a keys are fine.
E.g. if you had to complete the ICC heroic trilogy to attune for the ICC raid. It makes a lot of sense, adds to the lore, provides much-needed gear, and it's a reasonable difficulty level.
If you can't master the tactics for the heroics, then you're going to wipe miserably in the raid anyway.

When they removed the attunement for Karazahn, it opened the door to all the moaners and whiners who felt they 'should' have raid-spots because they were level 80's too.
At least the attunement process meant that anyone entering Kara had an idea of the level of difficulty to expect, and had already acquired at least 'some' team-play experience after 80 levels of solo'ing.

And (another pet hate of mine) it meant the PvP kiddies had to learn to play PvE style before they could come and ruin things for everyone else.