Since the WoW devs have spoken that they'd once again amp up dungeon difficulty? (Lulz them and their lies) I wrote a topic regarding it in their forums, providing the insight only a noble person such I can. Since this is the discussion regarding what's next for WoW, why not provide my highly monocled opinion here as well? :O
http://eu.battle.net/wow/en/forum/topic/9052187924#1
Dungeon difficulty is something that is often brought up, both in threads and in form of threads such as this. More often than not the views are quite simplistic too, such as the demands for more CC.
Those demands often fail to dig deep into the reasons of why CC was useful in the past and why it's not anymore, with prevalent opinion being "Increase the damage so you must CC! Add more HP so you must CC!" sort of stuff which is inherently wrong and only additivies to the difficulty if paired with real reasons for crowd control that existed.
Far too many things have changed mechanically to make CC itself interesting enough to spice up dungeon runs as large majority of reasons for it has vanished, few of them which I list later.
To understand what made dungeons difficult was not the damage necessarily, nor the HP of the mobs but the various mechanics they laid down which do not happen anymore, mostly replaced with clearly marked and EASILY avoided "AVOID THIS STUFF IN THIS HUGE RING OF WARNING" or "INTERRUPT BLOODY NOW!!!" sort of stuff.
To make dungeons difficult again or atleast mildly challenging, on any sort of level few key things needs to revert and be made.
To name few, crowd-control itself needs to aggro again so it becomes a reactionary, planned thing, be it as a puller or post-pull mechanic you apply to encounter, not something you can passively hold up almoust indefinitely and just wait about for pull to happen.
Threat needs to make return so people are more careful with how they play and approach the nuke in the first place. This very much includes healing threat. It'd add extra emphasis on usage of especially short term CC on fly if for example a long term CC target is freed but nothing is done to it.
Groups of mobs need to have back meaningful abilities that take direct control away from player. This means fears, gouges, sheeps, silences, mind controls and all sort of other stuff. This encourages creative play under unforeseen circumstances and promotes skill. A great example of this was the disarming mobs in BRD prison section. They disarm tank = practically no threat generation to any of them and those packs could be 4-6 mobs large. Demanded participiation from all group members in some form to handle them.
Mob group placement need to be more aggressive, not pre-chewn and conveniently placed. Packs that are closer together, and have patrols walking between them promote observation and more careful approaches to the pull itself. Stealthed mobs placed on surprising places can add a lot for good design.
These are few things that would immediatly make dungeons far more interesting to run again and now we're only talking about things that already have existed in some form within the past. Combine them with some of the new mechanics and we'd be facing dungeons of both surprising challenge which require some sensible thought put into them overall. It'd also promote a more hybrid gameplay from 5 mans, something that has practically not existed since vanilla and early TBC.