World-of-warcraft – How long does an expansion activity surge last

To be honest I think it's a bit too early to say right now. On the PTR even in heroics tanks are getting beaten up pretty hard, as they scaled up monster damage output, so the numbers will probably get rebalanced a bit before the patch goes live.

The biggest thing to get your head around right now is the revamped system of every healer having the three base heals - the slow, small efficient heal, the slow large heal, and the fast inefficient heal. They apparently expect healers to use the slow small heal when there is not much else to do, and to balance the other heals with their cooldowns during heavier damage.

Also, right now I'd pay more attention to what Blizzard says about healing than what the experience is like on the PTR. Here are some recent examples:

• State of Holy Paladins
• Holy paladin nerfs
• On healing difficulty

The reason I say to watch what they say more than what they do is, they've been very clear that if the PTR behavior doesn't match what they have in mind, they'll tweak the numbers until it does. In the last PTR patch most healers just spammed their slow, efficient spell because it was getting the job done... so in the next patch Blizzard nerfed those spells, as they want them to be only a fallback and not strong enough to stand alone. That sort of thing may well happen once or twice more before the patch goes live.

Incidentally, I find that checking the top page of mmo-champion to be the easiest way to keep track of Blizzard's position on things.

1) Most classic achievements aren't going anywhere with Cataclysm. Of those you cited, none are going anywhere, though the requirements for some are changing (If you already have them, you don't need to worry about it, but if you don't, you'll need to meet the new requirements. In some cases, these are actually easier than the old ones - Zul'Gurub is being removed from Classic Raider - in others, it's essentially resetting all your hard work on, for instance, Loremaster, if you don't finish. Some other changes:

• Did Somebody Order a Knuckle Sandwich and Master of Arms were changed to Feats of Strength in Patch 4.0, with the removal of Weapon skill. If you missed out on these, it's too late already, sorry!
• The Green Hills of Stranglethorn, and with it, the collected Nesingwary achievements, are - somewhat surprisingly - not going anywhere.
• With the removal of Zul'Gurub, The Deadliest Catch - will become a FoS, and no longer obtainable. Similarly, if you have any interest in a rare Raptor or Tiger mount, with it's attendant FoS, you have about a month left to farm one up.
• Zul'Gurub is also where you go to farm rep with the Zandalar Tribe, whose reputation is being removed with the Cataclysm. You can also buy this rep off of the AH in the form of a whole lot of token turn-ins.
• Since you asked - the Hydraxian Waterlords, (and Molten Core), aren't going anywhere. You're welcome to keep farming that rep well into the expansion.
• Finally, while Insane in the Membrane is not being explicitly removed, several of it's requirements will become impossible after the Cataclysm. Notably, there will no longer be any way to gain Bloodsail Buccaneers Rep (which also means if you want that Bloodsail Admiral title, you're running out of time for that too...), and the Librams needed for Shen'dralar rep no longer drop anywhere, and the NPC's to turn some of them into have disappeared. Ravenholdt will still be doable, but significantly more grindy, as the Syndicate mobs which you used to be able to kill to get to Honored have been removed - it's Junkboxes all the way to Exalted.

2) No. When the Shattering happens, it happens for everybody, regardless of their sub status. The world changes will be reflected in the vanilla game, and not buying the expansion is not going to buy you time to work on old achievements. The clock is ticking!

3) There are no TBC achievements changing as far as I know, but several Wrath raiding achievements are going to get the FoS treatment, since attempting them at level 85 trivializes them. These include the various ToGC tribute runs, The Immortal/The Undying, Champion/Conqueror of Ulduar and He Feeds on Your Tears

An easy way to check on future availability of any achievement in which you're interested is to look it up on WoWHead, and then click on the "Cata" link in the top left corner, which will show you how it is being changed in the Expansion (if at all). If it's been recategorized as an FoS, or had it's criteria changed, you should see the difference.