"Every time you earn an achievement, WoW tells people about it. Anyone in your guild, anyone in your group, anyone even standing near you, no matter their faction, will receive a message - "[Tieria] has earned the achievement: [Tastes Like Chicken]!" Friends can whisper you their congratulations."
You can turn this off. I did - I found it annoying. I really don't care when the 60th person logs on and gets their pet, or getting my chatbox flooded whenever I down a boss.
"That dirty Horde can know right before his spirit leaves his body that yes, he was your 10,000th PvP kill. And if you're dedicated enough to earn a server-first achievement, the message will be broadcasted to everybody. Yes, everybody. They'll all know how much ass you kick (or how much free time you have on your hands)."
You really need other people to know about your accomplishments to enjoy them?
Back in my day, I got every poe in OoT just for the sake of doing it. ;P
I just don't like how Blizz arbitrarily raises gameplay time by making you do things differently WHEN YOU COULD HAVE JUST DONE THAT ALREADY.