Althought it's only July (tomorrow), and some might think it a little early to start thinking about it. It's about now that we face the Pre-Expansion Blues. The "im loggin until Cata/TBC/Wrath gets released"
So between now and then, is the ample time to finish off what you have left behind, the loose-ends of achievements.
Although we have cleared Naxx, we've never really had a full clear on farm, achievement wise, i think alot of folk would like that as an achievement. Ulduar is the same, however will require more hard work for Yogg, since we haven't downed him yet.
There's an urgency for badges, weekly raids and achievements by members so they are able to either pug with the achievements or what have you. And also that sense of "I've cleared that with my guild" attitude.
Personally, i would like my full T10, and gear level up to scratch before Cata comes. Ok, i might not get 25 man level gear, but T10 is enough for me to screen shot, and say....i managed that pre-cata, not post