What's wrong with having a few achievements that are really hard to get? Do people just want every game to be Naruto (or whatever that was) and give you 1,000 points in a few minutes? I am angry that Dirt 2 really doesn't have any skill based achievements and you can get 1,000 from just grinding. I am not proud of my 1250/1250 on Oblivion but I am proud of my 200/200 on Castlevania SoTN. I'm proud of my 1060 on Mirror's Edge.
What NoThru22 said (except it was some Avatar game, not Naruto, as I recall). I think it's great when there are some achievements (skill-based ones, that is, not grinding-based) that a good number of people aren't going to get.
I have no issue with a challenge but the stark contrast with the rest of the achievements is a bit overkill.
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Its aimed at the players who want to prove they play games all day. xD.
I personally welcome Hard Achievements, but there's HARD to get, and Time Investing ones, AND THE ONES THAT YOU HAVE TO HAVE YOUR FRIENDS OVER AND 3 FREAKING MICS!
The only time consuming achievement I'm going for currently is the Zombie Genocidest on Left 4 Dead.
To answer your question why they would put a really hard achievement in a kids game is because most kids aren't as into gamerscore from what it seems to me, they see achievement unlocked and are happy for a minute then forget about it later.
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And the 100% solo thing is understandable. You can't make getting 1000/1000 (or Platinum) that easy can you? It was probably more of a jab at the veterans coming in thinking, "oh, this game should be easy to get all the achievements/trophies on" then see the 100% solo award and its like, "damn thats gonna be almost impossible"
But I think if anyone practices enough at it its very possible. I don't plan on getting it anytime soon, but its a goal to set sometime. Maybe next year when I can actually hit those zig zag HO/POs better.