Instances are Hard!

I've been reading a lot of forum threads about instance difficulty level.

I'd be willing to bet that the majority of those complaining about it's difficulty are trying to skip a step (or two) in their gear progression and are not doing what is within their power to help a group succeed.

1. Skipping a gear tier
Remember - we had no LFD tool in BC or at the beginning of wrath when we were all grinding 5-mans for adequate gear to succeed in raids. There was nothing telling us when we had sufficient gear to complete instances. Now that we do, people assume they're ready because their average Ilvl of the best items for each slot in their possession is this or that. Individual readiness is not solely dependent on your average Ilvl - more-so for some classes and specs who seem more gear-dependent than others and for whom stacking particular stats early on makes a world of difference.
Just because you CAN queue for normals doesn't mean you're ready to.
Just because you CAN queue for heroics doesn't mean you're ready to.
Just because you have all heroic-level gear doesn't mean you're ready to raid.
If you're sitting in all obtainable pre-heroic gear (all 333 with some 346 from revered) and still having issues in heroics, there's something else wrong (see below).
If you're sitting in all obtainable heroic-level gear (all 346 with crafted gear and exalted rep gear) and still having major issues with the first raid bosses, there's something else wrong.

Confidence is a pretty big contributor to group success and confidence comes from gear-readiness, encounter knowledge, and firm grips on class mechanics. Not only must you have gear to be confident, you have to practice the encounters enough to know what to expect and you have to know your class well enough to know what you can do to aid in your own survival or to help your group succeed.

Before stepping foot in a raid in any expansion, I have always been wearing the highest and most suitable gear available to me via any avenue. For me, this was a way to tell my peer's i'd put the work in and deserved to be there along side them. Not everyone needs to go this far, but nobody should complain about difficulty level when part of that difficulty would disipate with proper gear.

I didn't do very much raiding in Vanilla - was just filler for AQ and did strat and scholo when they were 10-mans.

In BC, I was in nearly all epic gear before stepping foot in Kara (did the normal grind to get gear and rep to do heroics then did the heroic grind, crafted what i could, and grinded all reps to exalted) and I still found a few of the bosses in there to be really annoying. Moroes, Curator, and Prince all took about 3 weeks for us to kill. After everyone had everything (except the Spite Blade) they wanted out of Kara, we moved on to Gruul, who took us at least 2 months to kill. We skipped Mag until we were in SSC/TK gear because we were disorganized. SSC and TK both fell very quickly with the exception of Vashj and Kael, both of whom took us a month of concerted effort to down. We then moved to BT and Hyjal. Before the Nerf, we cleared Hyjal, but Archimonde took us a good month of two. The hardest encounter for us in BT was Reliquary of Souls, which took us another month to kill.

In Wrath, again, I was in the highest level and best-suited to my role gear I could get my hands on before stepping foot in Naxx. I went through all the normals to get the gear I needed then all the Heroics to get the gear I needed. Of course, this gear grind didn't seem that necessary in retrospect because everything in Naxx was a total cakewalk (except Thaddius when the lag was really bad there for a while - we called him the lag boss). We then went into Ulduar and lost 4 solid raiders upon wiping on Ignis and Razorscale a combined 8 times. I never got to kill Yogg or Algalon because my guild decided to switch to TOC upon it's introduction, but I suspect they would have been challenging. TOC was comparably easy and, with no trash whatsoever, didn't take more than a night to clear. We moved to ICC and amongst some guild fraying, we started doing 10-mans instead of 25. ICC, trash aside, was either difficult or interesting enough the first kill of everything, after which, it became a grindfest for main toons, alt toons, super sucky bank toons.

In Cata, again, I did the normal circuit then the heroic circuit now the raid circuit. Honestly, I'm finding the raid bosses MUCH easier to down than those in BC or Wrath, with only 1 or 2 nights spent learning the encounters before downing the bosses. I attribute this to the heroics being more of a training ground than in expansions past and to having a lot more collective experience on the toons I play. I run a casual raiding guild wherein we have about 5 players who we can count on to do what they can to help an encounter run smoothly with the other 5 spots filled by people who have shown an ability to learn from mistakes and work as a team. Magmaw took us 2 nights of learning, Omnitron - 1, halfus - 2, Valiona - 2, Conclaive of Wind - 2. 5 raid bosses down in less time than it took us to kill Moroes or Ignis or Putricide and in 1/4th of the time it took us to kill Archimonde, RoS, Thorim, or LK. This does not bode so well within the argument that this xpac is hard. I use the bosses above as examples a lot when newer players start whining or sighing openly upon experiencing defeat for 2 days - 2 days is NOTHING. When we get to a boss that requires both gear and coordination, I fully expect it to take us 5 or 6 days of learning based on our experience so far, but I wouldn't say it's hard until we wiped on it for a month.

It's important for people to realize that Bosses aren't meant to fall at their feet and hand out epics (anymore) - if they were, they would not be bosses, but rather some at-level elites taking a stroll in a world of daisies and candy canes.

It's also important for people to have reasonable expectations given their level of gear and competence. If you're not in appropriate gear and don't do your best to help a group succeed, failing should be expected, not an impetus for unreasonable QQ in the forums.

2 - Helping a group succeed
There are many things you can do to make everything run smoothly. First and foremost - view heroics as training grounds for raids and normals as training grounds for heroics. Learning how to use your classes' abilities to control your environment and keep you and others alive is part and partial to your success and that of your group.
You don't have to hone these skills in Heroics, for the most part - You can (and should) practice in normals. Our new Mantra's are:
"I have an interrupt and I'm not afraid to use it"
"I have cc and I know how to use it"
"My mana is precious, I shall use it wisely"
"My cooldowns need more down time"
I'd suggest running normals until your most important abilities become second nature. No, your damage output is not your most important ability - it will always be secondary to your ability to play your class well.
The mantras really say it all.
If you are a tank, you might have an instant queue and feel a power to do whatever you want and be as much of a jerk as you want, but you are still responsible for doing what you can to help especially your healer out - you have many damage-mitigating cool downs - start using them.
If you are a healer, you are now in resource management mode. To give your group less down-time between pulls, try to be efficient - not everyone has to be at full health all of the time. 1- they won't notice and don't care as long as they don't die. 2- most pauses between pulls are for healer mana. Blizz has given healers choices as to how to use their mana. All healers have a quick heal, a big heal, an efficient heal, a group heal,etc... knowing what heal to use when is important and using your mana regaining abilities freely will make a big difference.
If you have an interrupt, keep it on cooldown to mitigate the damage going to whoever is targetted by the interruptable (thus avoidable) ability. Just because it's not usually targetting you doesn't mean you shouldn't interrupt it. It's a global cool down used for group success instead of extra dps. Nobody cares how good you look on the meters if you're not doing your job. If the tank dies because you didn't interrupt, that wipe is YOUR fault. I always thank the pugs who actually interrupt because all 3 of mine have been on cool down when I'm about to get wtf owned by some special ability - it saves my life, which, in turn, saves the group from a wipe. Lack of interrupts is a major contributor to instance difficulty. Yes,it's going to hit you for 95,000 if you don't interrupt it and 0 if you do. It's a no-brainer, yet people seem to have some aversion to using their interrupts.
Crowd Control is an obvious choice for mitigating overall damage - if it's incapacitated, it can't hurt you (or anyone else). Most CC happens before the pull, but there is no law against using it in the midst of battle. If a mob is headed for one of your group members - regardless of fault or egos, cc it. I know - if they're dead, you'll look even better on the meters, but with anyone dead, a group as a whole has less chance of succeeding. Just incapacitate the bad guy until the tank picks it up.
Aggro also has a very important spot on the helping a group succeed list, but I won't expound on it too much here (made a whole blog post about it). I'll just say this: If it's hitting you, stop hitting it and use any aggro dumps you've got. If it's about to hit you, stop hitting it. I know it's a common theme, but you will look worse on the meters if you're dead. You are just as responsible for keeping your threat in line as the tank is. Yes, the tank has 1 or 3 taunts, but they are likely on cool down if mobs are peeling off her (or him), so do what you can to survive - don't keep nuking (at that point, it's no longer even remotely the tank's fault if you die).

TLDR:
If everyone looked at pugs as an opportunity to practice placing group success over personal gratification and had appropriate gear and observation skills, they'd all be much easier. It's not the encounters, it's the people in them.