Just because I'm one of those crazy gamers who feels the need to completely and totally beat the snot out of every game I play. I don't do it so much for the points, because I could really, really care less about a number, but just as a checklist of sorts. If I can get all or close to all of the achievements, I've played the game to it's fullest.

Achievements in video games or almost anything except real life I think are arbitrary and possibly demeaning to those who are newly joining. My 950+ post rank is something that someone is probably coveting, and I find is absolutely arbitrary.
If I was feeling particularly intense on the same opinion, I might say "Why not make the Rank say 'Better than Thou', or 'Superior'?".

I saw it coming if I was to get banned for this.

they are a measure which allows you to say "i am a better console gamer than you". which is great until you have had you arse handed to you after playing the game for 60+ hours straight thinking you are a pro. *dies a little inside*.

Same as all of the above. it also just makes me feel like im not suckin at the game an makes me wanna keep playin it. gamerscore means buger all. me an my mate try to beat eachother but only for a laugh.

Oh, I don't want to be off topic but I fail to find this in any other threads/sites/search engines.

Are there any achievements that have an in-game effect? Not achievements that are actually in the game (CoD4 challenges per-say) - but an achievement that you completed, and then because you completed that certain achievement, you got something more than just 10-100 points added to your score.

ElArabDeMagnifico:
Oh, I don't want to be off topic but I fail to find this in any other threads/sites/search engines.

Are there any achievements that have an in-game effect? Not achievements that are actually in the game (CoD4 challenges per-say) - but an achievement that you completed, and then because you completed that certain achievement, you got something more than just 10-100 points added to your score.

Yeh. In Mass Effect, getting a '[insertpower/weapon] Mastery' allows you to create a new character with that power/weapon as a free point slot. Eg. Say I get Assault Rifle Mastery. If I create a new Vanguard character, then I can give them the ability to use Assault Rifles.

Get it, or am I really bad at explaining things?

I like getting 100% completion. Anything else is a bonus.

I like them for proof because I only do the crazy hard ones (like beating ppl online in GH with a standard controller I mean that took so long!)

Just shame they're not like the TF2 achievements, where for a certain number of achievements cleared, you get an option of a new gun, not necessarily BETTER, just different.

It means they're adding new content, without wildly unbalancing the playing field for new people.

Note the above is opinion not fact, now feel free to flame me about TF2 balancing issues and how the axetinguisher is SOO much better.

Wouldnt it be nice tho, if once you'd cleared say half the achievements in a 360 game, a new level opened up, or you got a new weapon to play with?

I only do the oddball ones, or ones that sound cool. Like, light 5 enemies on fire at once. Or kill 2 enemies with one attack. Stuff like that. I never do collectibles unless I absolutely have to.

stompy:
This:

Strafe Mcgee:
Usually because they're fun and it gives me different challenges in games I enjoy that I want to complete.

thebobmaster:
to prove to myself that I am capable of it.

and

GloatingSwine:
Because they are shiny. And I like shiny things.

Yep.

Normally? Because I'm just trying to play the damn game.

"Let's see, that's Steinman down..."

BOINK! ACHIEVEMENT UNLOCKED.

"Bwah, what the deuce?"

I like the little "ba-doop!" sound it makes, and I use it to see what my friends accomplished in the game. The number total isn't that important to me.

I see them as challenges, take the "mile high club" achievement from COD4 for example.
I spent almost 2.5 hours on that one, but I did it, and it felt great.

I also really like the OP's idea that you should be able to use the achievement icon as your gamerpic, some achievements are rediculously hard to get, and you should be able to show them off like that.

Because they're there.