MMOs: making it difficult and unrewarding to group, particularly with friendstldr version:
The reliance on quest completion for rewards, the separation and stark contrast between group and solo content rewards and difficulty, and the effect a significant level difference between group members has on difficulty and rewards, all combine to make playing most MMOs with a small group of friends a real pain, especially (but not exclusively) if they have significantly different gaming time available.
War & Peace version:
I find most MMOs, especially recent ones, it's quite difficult to do much in the way of grouping with the handful of friends I have, and when we do it's not terribly satisfying. This is usually the result of a few different factors:
Most of the content leading up to level cap is designed around the solo player. Now, don't get me wrong, I don't want to discourage the solo player and I like to solo myself. The problem is this solo content will be mildly challenging for a solo player, but even adding one player to the mix and it's massive overkill, removing any semblance of danger or satisfaction. It turns into a race to be the one to attack first, with the slower player never actually getting to fight anything. Worse, many games don't have good systems for splitting xp and/or loot, such that if you run a given quest chain with a group of e.g. 3, you'll all have less xp and/or loot at the end of it than if you'd run it solo.
Corollary to that is that group content by contrast typically requires a full group and that said group comprise an optimal configuration of classes/roles, else it's doomed to fail. So that 3 person group that's overkill for all the solo content probably isn't going to cope with the group content. There's usually not a middle ground.
Secondly, modern MMOs tend to be very dependent on quest chains. These quest chains usually require that you've completed previous quests to get the next one - so better not skip/miss any or you're going to have to go back and do stuff you've outlevelled and aren't really getting either a challenge or a reward for. They also tend to require that in a group you are currently on the active quest to qualify for the xp/loot reward at the end - and modern MMOs tend to *heavily* favour quest completion as a source of xp/loot (presumably to discourage farming/powerleveling).
The problem with this aspect is that if you're playing with a group of, say, 4 friends, the chances are you're not all going to always be playing at the same time. As a result, quite quickly you end up at different levels than your friends, and at different points in the various quest chains. The friend that's playing the most then has to choose between playing with the rest of you and not getting any real rewards/progression (because they've already done those quests) and having to go solo to run the quests you're not ready for. Meanwhile the friend that can only afford to play occasionally quickly lags behind and while you can maybe carry him/her along with your group, they're not going to get much actual progression because they're not eligible to take those quests yet (and so don't get completion rewards for them).
Finally, tied somewhat to the second issue is that in addition to being at different stages of the quests/content, a difference in level will quickly develop. If one player has leveled a way ahead of the group, the rest of the group are too low to join him/her on his/her quests, but if he/she tries to run with the group on their quests it'll remove any semblance of challenge due to the level difference (i.e. bringing a level 30 along with a level 15 group on level 15 content). Indeed, depending how xp is calculated, having that higher level player along may well kill xp/loot for the entire group.
The solution to this is scaling content - have the strength and/or number of enemies scale to the number of people in a group (admittedly this is easier to do with instanced content than open world, but you can do it to an extent with open world). Generally ARPGs have managed this, but only a small number of MMOs seem to even attempt it. Furthermore, you need some system to scale a player's level to the content they're doing (either scaling the entire group to the zone they're in, or having the group scale to the group leader's level) so a high level player can go back and do low level content (and still get both a challenge and rewards) and a low level can do high level content (ditto). There's a few more games that do this to some extent, but still not enough. Very few seem to do both.
Basically I really miss City of Heroes