The big difference between gear and achievments is that unlocking achievements has no in-game consequences. Congratulations, you ate 50 types of food, here's your rare mount!

And don't tell me the new raids aren't accessible - there are successful PuGs everyday for Naxx and Obsidian Sanctum. This is so much better than the days of Pugging UBRS and failing (when it was 15 man).
Except the major difference is that now, if you kill incendius, you're locked to that instance of UBRS for the whole week. Pressing concerns, bad puggers, people dropping can all ruin your run, and then you're stuck with your thumb up your ass while you wait for the reset timer, or someone else saved to the raid pugs the rest of the bosses, or any number of other things that can ruin your once a week shot at this stuff.