This is fine, and it's part of the moving on process with content: if you're regularly farming M'uru you probably don't need anything off of Mother Shahraz anymore except for maybe bad luck on shoulders. Some guilds were fortunate (or unfortunate) enough to be completely done with Black Temple and Hyjal well in advance of Sunwell Plateau. Others are just now finishing up with Gruul and Magtheridon and preparing to move into SSC and Tempest Keep, while still others are just now done with those.
When my guild finally killed Kael'Thas and Vashj, there was a simultaneous rush of joy and sense of completion and an immediate desire to move on to the next raid. We ran right off to Hyjal as fast as our feet could carry us (well, we used portals too) and threw ourselves at the trash, deliberately not even reading up on the fights. We just wanted to see it, to see the new content that had been just out of our reach. At the same time, though, we knew people still needed vials and some wanted to complete their T5 sets, so SSC and TK stayed on the raid menu. By now though, it's clear that time is coming to an end.
I'll miss the Kael fight, as a tank. Oh, I hated him. Passionately, like fire. I hated having to have the shield before he came out. I hated having to time a nightmare seed or Last Stand just so for the first pyro so that I could eat the second one with the shield. And yet, all that tightness, that absolute lack of margin for error is what I will forever miss about the fight as we finally leave the Tier 5 instances behind. We don't raid nearly as often as some guilds, two to three times a week maximum, and we have a smaller core of raiders, but I've gotten to experience this content with good players and it feels slightly melancholy to realize we're leaving it behind forever. I'm excited to move full time into Black Temple and Hyjal instead of just picking one of them for the week, but sad that I never saw the Pauldrons of the Wardancer drop and now I never will.
As always, gentle reader, share. This has happened at least twice in the life cycle of the game, and with Wrath of the Lich King on the horizon, it will happen again. What content do you most remember fondly? Which zone, instance or raid did you remember leaving forever the most strongly?