Many of the open world game these days have an excellent story-line and a variety of stuff to play and do in the game. But they also have these humongous list of side missions to complete, objectives to achieve and collectibles to collect. I agree that they add more to the game in terms of gameplay, but it would be far better if they were reduced and integrated into the story itself. Not all players have the luxury of time to complete such tasks and often they are left unfinished once the main story missions are completed.
Examples[]
1. Assassin's Creed III[]
The link here gives a comprehensive list of the objectives and missions you need to complete in order to achieve 100% completion in Assassin's Creed III.
2. Batman Arkham City[]
There are tons of collectibles in the game and they are scattered all over the game world. Some of them are well hidden in places you would never see in normal gameplay. There are just too many question marks that it eventually became frustrating. Especially for players who want to complete all achievements, having to go to each question mark and solve puzzles over and over again is just boring and time consuming. This mechanism has nothing to do with replayability. It just exists to drive players crazy.