You are not an idiot. Its a good question. The idiot is who says you are idiot!
Anyway my point of view:
- In basics Achievement idea is good. You can redo something you are already bored of, do on a more difficult level, and feel you have done a hard version of encounter. All in all, everyone will be able to down the same boss, less will do achievments.
- The execution of Achievements are not so good. Some achievements are plain easy and even more easier in dungeons. You don't even think about it, and you complete it as a casual player. Maybe the idea behind that was to slowly introduce achievements to casuals too. However the BIG BIG downside of that is when you do a hard achievement, you get the same 10 points for it than an easy one. Why call the easy one achievement when everyone can do it without any focus? There is a BIG spread about complexity of achievements (doing 1st and 3rd in Azjol Nerub... vs 2nd achievement there). Therefore this comes as a mixed bag, and you don't feel special about doing one achievement.
- Achievements - if you look at EVERY achievement - gives me an even poorer picture. A casual player who is running around like a headless chicken will have more achievement points due to eploration than a raider who knows how to play his character and adds in extra hours to learn and master encounters with his friends - doing the hard ones too. So with this view, achievements fail, as they don't distinguish players - just too many texts to check what a player did. Some might even feel they are forced to do achievements they dont want to - but they want to get more and more scores from the game.