HotS nailed this one:
-A daily mission which is random and unique to each player. That way you avoid everyone trying to do the same thing at the same time.
-Missions are simple. ‘Play 3 games as a support/assasin/tank hero’ or ‘play 3 games as a hero from x universe starcraft/diablo/warcraft’ or ‘win 3 games’
-Missions offer a substantial reward. The equivalent of playing 15 or 30 games in a day.
-Missions vary in work load (but the reward corresponds to the missions) so sometimes you have to play 3 games for it to complete, less commonly you’ve got to finish 8.
-You stack up to four missions. IE miss one mission or leave one incomplete then it stays until you clear it or it falls off the four missions slots. This is important! This means there’s no need to log in every day and complete missions. People have a life, work, family other hobbies, so if they miss a day or two due to being busy, then they can still work to clear their backlog of missions and still get all the rewards. This is way more lenient than swapping missions or something. Player’s aren’t employees, they just want to have a good time.
Daily missions that disappear are only suited for people with a lot of time on their hands. This system works brilliantly and isn’t punitive on the people that have a disposable income. And that’s quite a relevant point for a F2P.