There is an inherent limit to skill in World of Warcraft. Assuming a static game, at some point theorycraft will show the BEST way to play your character, the BEST way to set up your raid, the BEST way to react. Which is why the game isn't static. The game changes, and the way we play our classes changes. However, Blizzard tends to change small things, and rarely changes the overall direction of a class. As time goes on, we get faster and faster at returning to maximum efficiency after a change. While expansions with many new talents and a few new abilities cause us to shift models around and rerun our simulations, the actual effect on our characters tends to be minimal. As an example, Affliction was a powerful spec in early BC. It was overshadowed by Destruction later on, and with the advent of Wrath, returned to its former glory. But did the playstyle change? Ultimately it was still maintain DoTs, cast Shadow Bolt in downtime, Life Tap to avoid OOMing. The change to Drain Soul meant it became worth casting at the end of a boss fight, but that's really the only change. Even the upcoming patch won't really change the playstyle, it'll just switch up how many DoTs are on your target at once.
As time goes on, and our understanding of classes becomes more and more mature, more players will be at the limit of each class. Not everyone will become godlike masters of WoW, but each individual tends to increase in skill over time, albeit at wildly varying rates. So how do we suppress this constant increase? How do we make content difficult while assuming that players are constantly gaining skill? Well, first Blizzard introduced DKs, which was quite a challenge. DKs have basically been an arms race between Blizzard and The Community, wherein Blizzard wants DKs in a specific place in terms of tanking and survival, whereas the community wants DKs to deal more DPS and survive longer. Since the class is still very new, neither side had accurate models, and odd builds like 32/39 DW skyrocketed in effectiveness while tanking builds became largely overpowered. But that's just one arena.
By far the more interesting challenge is vehicles. Each vehicle a player can use is like playing a very tiny but entirely new class. Learning how to use a vehicle properly is Blizzard's intelligence test for the community. You can see this in the Malygos encounter. Many players cannot figure out how to properly use their dragons without outside assistance. Even with the entire skill set in front of them and time to look it over, many people cannot figure out what to do on their own.
The Oculus is an even better example. I love the instance, because you're confronted with the challenge of mastering a class that you haven't seen much and don't use all the time. You don't get to practice outside on the target dummies or solo to get the feel for it, you're in a group, with a variety of new vehicles, and a few enemies in front of you.
Because of the fact that vehicles are a nonstandard mechanic, Blizzard occasionally tries to be blunt about how to properly use each vehicle. For example, the amber (bronze) drake in Oculus has three skills:
Can you figure out, on the fly, without being told, the optimal way to DPS a boss, assuming you're in a group? This is a relatively easy one, but I'm always surprised at the number of people that can't work it out. The answer is to talk to another player on a bronze drake. First, you use Temporal Rift, while he uses Stop Time, then Shock Lance, and immediately hit Temporal Rift. This will give your ally quite a few shock charges. As soon has he has them up, and your Rift is up, he hits Shock Lance then puts up Temporal Rift. You alternate back and forth like this until the boss is dead. This can be done rapidly, without any voice chat, as you just need to make sure another player's Rift is up when you Lance so that you do extra damage and they get charges, and if you don't have any charges, Rift until you do. Assuming everyone knows the strategy, you can do absurd levels of damage this way, including a 5-bronze kill of Eregos on Heroic.
As I mentioned before, I'm always surprised by how many players simply cannot work that out. Person after person rides a bronze drake and spams Shock Lance, or just keeps Temporal Rift up the whole time without ever attempting to DPS themselves. This isn't just the case with pugs either; I've met many players in top-level guilds on my server that couldn't work out proper strategies for the drakes in Oculus and couldn't figure out how to heal or DPS on Malygos. However, I've definitely found a correlation between skill in WoW and skill on vehicles. Generally, the people who get vehicles right are the same people that don't stand in void zones, don't get hit by lava walls, and don't get shocked to hell on Thaddius without having to be told that THIS fire is just as bad as ALL fire.
Vehicles may be a silly gimmick, but they're a really solid way of separating the different levels of player in your guild and they're a fun challenge and an interesting dynamic for boss fights. Of course, considering the number of people that hate Oculus because it's "so hard," adding more vehicles to future encounters may not be the best business strategy. Ah well.