Regarding the TF2 achievements:
The community has been pretty split regarding the achievement reward system. The reason for this is as soon as they started giving alternative weapons away with achievements, there immediately popped up servers devoted entirely to grinding the achievements, because people obviously wanted the new weapons right away. This was okay in itself, but not really what was intended.
It also caused people to try and grind achievements on normal servers, thereby disrupting the flow of the game for the rest of the people playing. Not quite as cool. =/
In the most recent update, they switched the system, so that instead of getting the weapons by achievements, they appeared as random drops while you were playing the game instead. This, of course, made the Internet even less happy, and started complaining about how there wasn't any skill involved anymore, and how rare the new weapons were.
In the end, they enabled the achivement system again, so now you can get the weapons both ways.
So, what is best? Giving alternate weapons through random chance, or through a system based on skill, thereby only allowing a select number of people to get them?