"Let me know when you actually can get something for all those achievement points and I will start caring. Until then, there is better stuff for me to spend time on."
Such as commenting on things you don't care about?

Achievements are mainly just something for the "completion"-obsessed or people who want to get the rewards (pets/mounts/titles/misc items), whether it's for showing off or for the sake of collecting.

"Let me know when you actually can get something for all those achievement points..."

....such as a feature article on the biggest World of Warcraft blog in the world?

Believe it or not sometimes people do things for the challenge. The reward is the satisfaction of the accomplishment. If anything material rewards seems more akin to "epeen waggling" as then you have something showing off what you did.
Grats to Zoltan for such an accomplishment. Or maybe he has one major case of OCD lol. Either way wow!

So I guess you've never done a crossword because what's the point when there's no reward? Or you're quite happy to fill in just 1 or 2 answers and consider it complete?

It's a good job nobody thought to tell Edmund Hillary there was no reward at the top of Everest.

Some people like to do things just because they're there, some people like to complete things because it's their own mini-challenge.