Most raiders don't spend all the time they are logged on raiding and frankly, I find there's very little to do while I'm not raiding.

A trend I've noticed among my guildies since WotLK is that more and more only log on a bit before raidtime so they can prepare for the raid (get/make flasks, food and repair) and log off once the raid's over. Most did the Tournament dailies while they were new and got their Crusader title (if they care about the achievements or the title at all).

But what is there to do beyond that? We saw all TBC and older 5mans while we were leveling, by now I've gone through all vanilla raids and I did the TBC ones while I was 70 (Sunwell up to and including Felmyst) and while it's still fun to go to TK, BT or some other TBC raid it's just not the same. Most bosses can just be zerged without regard for the tactics taking out a lot of the fun of actually doing that boss.

WotLK 5mans are a chore, maybe I'm just on a sucky server but it's so stupidly hard to find a group for any of the more interesting 5mans that it often isn't worth bothering with (insane fun spending 2-3hours completing a group for Oculus only to have it fall apart after 30minutes of "Additional instances cannot be loaded"). I blame the sheer lack of useful rewards for raiders (which nowadays is about everybody anyway) for much of the lack of interest, gear up in heroics, jump into Naxx10, pug Naxx25 a few times and there's no more reason to do heroics (except to maybe gear up guildies).

Battlegrounds are another pain in the ass. Class balance is so totally screwed up it's not even funny anymore. Usually the games end in a gankfest, which isn't fun whether you are on the receiving end or the dealing end, unless you're only farming HKs I suppose. That is, if you are lucky, because half the time Horde starts with a lot less players than alliance in our battlegroup (appropriately named "Misery"), usually resulting in a lost game before the numbers finally even out. But hey, we all know only arena really matters anyway, right?

So, what do we have left. Leveling alts I suppose. Besides learning the particulars of a new class this suffers from much the same problems as the mains. Finding groups for instances while leveling is nigh impossible, everybody seems to either get boosted or zerg his way to 80. You've seen most of the quests before, of course you will be going to different zones than you did the first time (right?) but there's only so many zones (or alts) you can go through before you end up doing the same quests again, and again...