Overall it tends to come down to relative gear level. That's it. Just about everything in WoW can be beaten through gear, even if the fight is intended to measure something else.
Gear is not hard, gear is a time sink. For example, I think Naxx is too easy. But what would make it harder? The simple answer is to increase boss health and damage by 10%. Boom, harder, right? Wrong. All this does is tell the raid "go do heroics for another couple weeks and come back." It doesn't tell the raid "practice." No, it just says to go get more gear.
Practice and perspective also affect how hard something is. After you've done a fight a dozen times, it's not hard anymore. It's entirely natural and you'll think it's easy. A new player will think it is hard. I'll use this, at least for now, as my standard for hard: something which appears hard at first and gets easier with practice regardless of changes in gear. In essence hard is just a matter of practice.
Ultimately hard is, pardon the word choice, hard to measure. Why? Because we don't just get more practice, we also get more gear. Example time:
This fight killed guilds. It was the last 40-man raid I ever did on my first shaman. We wiped. Many times. It was hard. In contrast, earlier fights such as MC were easy, or at least easier. Domo, Garr, and Rag are the only fights I see as being actually hard, having mechanics which test skill which I'll call a combination of awareness, multi-tasking, and mobility. The rest tended to revolve around certain gear levels or repetitive tasks such as cleansing or decursing. But back to Razorgore.
This fight involves, or used to involve, a high level of skill from the entire raid. It required individuals to do their parts correctly but also the raid as a whole to work well as a single entity. There were people kiting and tanking and DPSing and healing and controlling the orb and any failure in any of those quickly wrecked the situation and led to a wipe. This was a fight which required a lot of practice.
Now though, gear has destroyed the hard part. Well, gear in the general sense of our characters gaining power. 20 levels and much higher stats have made the fight trivial. Now it requires barely any coordination, no kiting, and overall just not much skill. Now it's a simple matter of two people for the orb (one is enough if you're not unlucky), a healer for Razorgore, and some DPS to keep the adds from going after the healer. It is simple. The encounter which was once hard is not simply overgeared. I'd like to also point out that this is an even easier, simpler method than what I used at 70 when we actually had a tank for the adds because then we didn't totally outgear it. But even then, we clearly were overgeared for the encounter because at 60 you didn't have a single tank holding EVERYTHING.
Let's get back to Naxx. Heigan is a bit hard, right? I mean, you can't exactly overgear dancing can you? Well.. you can. Get enough health and powerful enough healers and you could just stand in the green stuff through the entire channel. We could remove the gear advantage and make the splash do a % of health, so two are a kill. But still, enough healing can overcome it. Make it a one-shot? That makes gear irrelevant, but also makes it very unforgiving to the slightest mistake.
Maybe the fight could be done somehow to not allow overgearing. Maybe every fight could be set up to not allow overgearing. But then what's the incentive for better gear? There are fights which don't test gear: vehicles the the absolute case. They tend to bother people though. They say they leveled their class, not a dragon. But is that the real problem? Maybe the true problem is that you can't overgear a vehicle. You can't farm gear and consumables and get through by brute force. I think that bothers people. It's not that they're bad or lazy, at least not innately so, but that over WoW we've been taught: if you can't do it now, get more gear. We don't learn to practice, we learn to overgear. When fights do require practice, like Razorgore, they often kill guilds.
Can it be any other way? If fights can't be overgeared, what is the incentive for more gear? The next encounter? It's likely that if a fight cannot be overgeared, it also cannot be undergeared; your gear doesn't help or hurt you on a vehicle. Maybe there could be the idea I'd suggested of gear caps: setting it so you cannot use gear from this or that raid while pursuing an achievement.
Maintain the difficulty past Ulduar. Add an iLevel maximum for the achievements and leave the mounts. This way you cannot trivialize them with gear because you can't get past a certain gear level. All Ulduar can give you is more experience with your class rather than a way to bypass the challenge.
Unfortunately this goes against the progression model which effectively amounts to perpetual obsolescence of everything which came before.
To truly test skill, WoW would have to eliminate gear as a major variable. Doing that would destroy the system which drives all progress past the level cap. In other words: WoW will never be truly hard, you'll just not have the gear for it yet. This doesn't mean that WoW is a bad game. It's been very successful and I enjoy it a lot. We just should manage our expectations.