I want a super-long dungeon in WoD, ~3-4 hours.

The amount of people who would leave straight away when they realised what dungeon it is...

Because what's a 15-30 minute debuff over a 3 hour dungeon.

Just like you get people trying to requeue or leave from dungeons like Wailing Caverns (I've had people do doing this when I'm levelling alts too many times for it to be a coincidence)

Even in vanilla people didn't enjoy doing full dungeons. You'd try and skip as much content as possible.

I'd be fine with Blizzard doing this (for those that want it) if there was a way to opt out of the long dungeons in the Dungeon Finder (for those that don't want it).

God forbid people should ever do things for fun in wow again huh? I will never get this mentality of just getting things over and done with for gear. Every part of a game should be fun and the kind of dungeons the OP is describing are what got me hooked on wow in the first place. Not to mention some of the friends I made whilst doing that content.

And finally, why not have such dungeons unavailable on LFD?

I think this is a great example of how different people play for different things. What people consider "fun" clearly varies, and we try to accommodate for everyone.

Many players prefer being able to run shorter dungeons rather than having to commit an hour or more to finishing a full run. Lifestyles these days aren't always very forgiving when it comes to time for gaming, so at least now with the shorter dungeons there's an option for people with less time, unlike before. While some of the old dungeons remain firmly among my favourites to this day, I can certainly see the benefits of having shorter runs available.

While I understand it takes longer to organise, there are multiple raids for those of us who are lucky enough to have extra time to spend on gaming, and there are more ways to find people to join you now that cross-realm raids are possible. The raid browser also helps you to find people who may be interested http://us.battle.net/wow/en/forum/topic/10637883888

Having said all this, dungeons in Warlords of Draenor will be a bit different to how they are now. Max-level normal dungeons will have better gear than levelling quest gear, and Heroics will be harder again. Be sure to try it out and give your feedback about WoD’s dungeons and raids once they’re available to test! http://eu.battle.net/wow/en/blog/11499600