I recently rented and played Operation Flashpoint: Dragon Rising and while I didn't continue after the first mission, I did enjoy the challenge and effort put into the game. But as a contempory game and a co-op game I found the experience lacking and it wasn't until later that I was able to pin down what it was.
Codemasters was making a follow-up to a game first released in 2001. While unfamilar with the intial game, I am betting that they included the major components of the first game. While trying to capture the essence of the first, I think they spent way too much time on looking in the past and not looking to the future. This game would probably have been received better had it came out years earlier. But games set in a open-world setting and include co-op are drastically different from their predecessors.
Player Rewards: The first instances I can remember go back to 2008, which isn't very far, where games (specifically co-op or open-world games) allow players to earn new weapons and/or customize their characters.
Army of Two, Call od Duty: World at War, and Far Cry 2 feature similar player progress. Although, Far Cry 2 and Army of Two had you purchase weapons with money you earned. CoD:WaW allowed players in single player, co-op, or competitve gain experience to go up in rank.
Since then Assassin's Creed II, Batman: Arkham Asylum, Prototype, and inFamous were open world games that gave similar rewards to players as they progress. Even the upcoming Splinter Cell: Conviction and Halo: Reach will feature similar Player Rewards.
While this type of system existed before in all different types of games. It has become an increasingly common feature and one could argue in certain situations should be a given.
I felt this missing in Operation Flashpoint, Dragon Rising above all else because as an game that takes usuage of an open world space in a military setting I felt no reason to continue.
I think while Codemasters was trying to make their game as much like the first, they forgot to take the license and just make it their own. Or simply name it something other than Operaton Flashpoint. Developers and publishers are at their best when they make the player feel justified in buying their games. Sure some people that have a core group of friends they can always get with and play co-op with could enjoy it. But for people they don't have that kind of consistent support games like OP:DR, where co-op is better than single player, fall flat for the masses who may not have the inclination to buy a game when they will rarely play it with friends.
Co-optimus.com is a great place where I have met a good number of people (and continue to do so) I enjoy playing with, but even then timezones and real life become obstacles around gaming. None of my gaming friends live near me, so most of my gaming is online.
So when developers do not design their game around all instances of it modes, be they single player, co-operative, or competitive, if they focus more on one than another. There will always be something missing. And while most times developers do have to choose where the majority of their time is spent, the best games offer balanced experience for all.
So what features or concepts in gaming are just must-haves for you?