The first thing I want to mention here is that I love both Jetpack Joyride and Tiny Wings. I've spent many hours with each and there isn't a lot I would change about either of them. Except this one thing!
Both games are essentially endless running games in which you try to reach as great a distance as possible until you hit something in one or run out of time in the other. Both also use a system by which you are given secondary goals beyond the main goal of reaching a great distance. These goals aren't unlike your run of the mill achievements except that they are presented in bite sized morsels giving you a changing list of sub goals to shoot for during each run. Ok, this isn't the problem. In fact, I find these little dangling carrots irresistible! My beef is with some of the choices. Both games include some goals that urge you to essentially do poorly, be it reaching certain distances without "swooping" in
Tiny Wings or endeavouring not to collect coins in
Jetpack Joyride , these goals run counter to your overall goal of doing well at the game and, well, aren't that fun to attempt in my opinion. So, I say, achievements, missions, whatever, are awesome and super fun stuff, but don't ask the player to play your game poorly or play your game in a way that isn't fun... Call me a crazy old nutter, but I like my games to be fun.