Blizzard's decision to make all raids available in both ten-man and twenty-five man modes in WOTLK has been very popular. Apparently it's enabled a lot of small guilds to see content that they previously wouldn't have been able to access due to lack of numbers. Good for them.
However, for me personally this change has largely killed ten-man raiding.
As a member of a twenty-five-man raiding guild the way these things were handled in BC was pretty much perfect for me. We'd have three twenty-five-man raids each week, and on what were officially non-raiding days I was free to goof off with some friends in a ten-man, be it Karazhan or Zul'Aman. It was fun because they were completely different instances, so it was a change of pace in more than just numbers. As an added bonus a lot of the loot in ZA was just as good - if not better - than many drops from Mount Hyjal and Black Temple, so it also served as a nice source of extra gear. And once you had all the drops you needed, there were still the badges. Awesomeness!
When the expansion hit, my guild decided to give people plenty of time to level to eighty and the first official twenty-five-man raid wasn't scheduled until January. Of course many people were eager to have a peek at Naxxramas before that anyway, so we started off by doing the ten-man version as a sort of preparation. So far, so good - but then we started to raid the place on "heroic" mode and suddenly the incentive was gone. Where's the fun in spending three days a week raiding a certain content, and then doing it all over again with only slightly smaller numbers and without getting useful loot or emblems that you can actually use to buy upgrades? I suppose we could have used the ten-man to get achievements that were too hard for us on twenty-five man, but I feel a bit "meh" about achievement raiding, so... the ten-man raids just died. Recently we've been trying to get ten-man Ulduar going again, but it's hard to motivate ourselves.
Also, in an interesting turn of events it's now starting to look like the ten-man raids might also start to hurt our twenty-five-mans. The raid force has been feeling the pain of the summer doldrums, with raids being cancelled due to lack of available warm bodies and progress grinding to a halt. However, there's a small group of people who are really into running Ulduar on ten-man... and interestingly enough they are available to run ten-man several days a week, while being mysteriously N/A for the main raids. Gosh, I wonder if they just don't feel like doing the same instance twice a week?
It seems that there's no way to win really, if you focus on "heroic" mode, normal will feel redundant and if you focus on ten-man, twenty-five-man will feel repetetive. I really liked being able to do both and it feeling different enough to be fun either way.