This post has been in the making for some time now. Ever since the discovery of nature resistance gear a few weeks back, the revelation of tier 10 set-bonuses and the PTR-testing of the first few bosses, we've had a potential insight into a lot of what the instance layout and mechanics could turn out to be.
First of all, nature resistance, which has been revealed to most likely be linked with the Rotface encounter. I'm still not entirely convinced this is the whole truth behind this, though - we know there will be a green dragon encounter in the later instance, which is still untested. I would not be surprised if this turns out to be this instance's Sapphiron, though hopefully harder. Another option is hardmodes - double the oozes, double the damage, double the fun? The actual requirement of resistance gear to be able to even compete in a hardmode setting would be an interesting turn from Blizzard, and it would no doubt justify the potentially ridiculous* crafting costs of the items.
*This, of course, relies solely on the availability of Primordial Saronite. If it's another Crusader Orb, there will be a lot of tears in Azeroth.
Moving onto set-bonuses, we've seen a few that are so powerful it's borderline ridiculous. Take, for instance, the Rogue 2-piece: "Your Tricks of the Trade ability now grants you 15 energy instead of costing energy."Allowing even greater burst, not to mention building a higher raid threat roof, on what is already the most consistent top DPS class? Let's move onto 4-piece Warrior tanks: "Your Bloodrage ability no longer costs health to use, and now causes you to absorb damage equal to 20% of your maximum health." Granted, it's no Shield Wall, but they certainly make sure Bloodrage is converted from a helpful pulling mechanic to being close to a 4th cooldown. A lifesaver when you know that Holy Light will land in just a second. And of course Death Knight 4-piece, "Whenever all your runes are on cooldown, you gain 3% increased damage done with weapons, spells, and abilities for the next 15 second." Now, I'm very excited about this, and how specifically it'll work. Will it, at the end of the duration, reset and start again next time you're out of runes? Will it have a constant uptime? Internal cooldown? In any of the above cases, this is an amazing DPS bonus.
What I read from these and other set bonuses, which by far overwhelms a lot of what we've seen in T8 and T9, is that we're in for a demanding ride. A few of these effects will trivialize even certain Ulduar hardmodes (Hodir, Thorim, XT, maybe even Vezax), Sarth3D-zergs will be a (slightly more) common PuG on most servers, Loken will be down before the second Nova. Upping the powerlevel on players this way (especially if Blizzard stays true to their pattern of allowing the 2-piece bonus to be available for emblems) either means they want exactly that to happen - lower tier hardmodes becoming more easily available - or, hopefully, that Icecrown Citadel will require these levels of DPS and tank control.
One final point before I conclude: the blue posts on crowd control suggest a nail in the coffin to the zerg mentality from Naxx and heroics. I really, truly hope this is the case. Hard bosses, tactical trash and encounters requiring both thought, skill and insane gear - this could hopefully be another Black Temple. The leaked graphics preview suggest nothing less.
Unless, of course, there is jousting, in which case it will be the worst instance in the history of the game. On general principle.
In other news, I've currently taken up the mantle of protection Warrior again, as my guild were having both tank and sunder trouble. I'll still be an angry Death Knight, though, only not as much. Expect posts on warriors to pop up there and again though - it's pretty much inevitable ;)