I think MMORPGs should be designed around encouraging grouping but not forcing it and that's a tricky balance. I'm mostly a solo player myself in the sense that for most of the time my *thing* in these games is exploring around with a ranger type char and that tends to be a solo occupation but I like a good group dungeon crawl now and then but they don't seem to happen as much in the current games. I also like the immersive aspect of these games so I'd want solo missions that fitted certain character types, for example:
Scouts getting missions to patrol the frontier.
Rogues getting sneaking missions.
Mages going to a famous library to do some research.
Priests getting a mission to bless a shrine in a village.
So I'd have solo missions like that but anything that would involve a lot of fighting would be more designed for groups. Warrior type chars might have a chance of soloing those but it would be very risky and much easier with backup. I also think dungeons should have sections for both. They should have sections that can be soloed and a deeper dungeon crawl bit with a boss for groups. It's important for dungeons to have soloable sections of the same level range as the dungeon. That way individual players will go there knowing they can solo but LFG when they get there.
The trouble with grouping is that the good thing about it is other people but the bad thing about it is also other people :)
This can be erm... pretty finely balanced a lot of the time so I think the game design needs to avoid any extra disincentives, for example:
Each player should get exactly the same exp from a mob solo or grouped. If a mob is a 100 exp then one player gets 100 exp and six grouped players get 100 each.
Each player gets to loot. The mob doesn't drop x and the first to loot gets it. Each player can loot the mob once and there's a random roll for each player. Exact same chance of getting loot from a mob solo or grouped. Except of course you couldn't have killed it solo ;)
I think loot is particularly important as not only does it stop some ppl wanting to group for practical reasons; they'd rather have a 30% chance of killing a mob with a 100% chance of the loot, than a 100% chance of killing a mob but with only 15% chance of getting the loot. But also loot-greed is one of the reasons ppl behave badly and ppl behaving badly is one of the reasons for not grouping. So reducing the loot-greed will improve ppl's behaviour in groups imo.
In cases where players have to kill a specific mob then all players should get the quest updated and not have to kill the mob multiple times. Of course this would already be covered by the loot rule if the mob always dropped "chief blugblug's ear" for each player when it died but in any cases that weren't covered by that then everyone should get the update.
The formation of groups should be as convenient as possible. One thing would be for the group leader to have a "warp to group" option so if there were three players in a dungeon and someone logged on LFG then the leader could bring them to where the group was. Generally the better and more convenient the LFG tools are the better.
Too many quests:
This may be just me but I find with the current games you get to a hub and pick up 20 quests at once and I start to get obsessive about trying to clear some of them because there's so many. I won't talk to anyone until I've cleared a few.I think I'd have it so you had one active mission at a time. You could have a solo mission and then join a group one and your solo mission would gray out till it was over.
I think it's a mistake to make certain types of char neccessary in groups and especially to have multiple neccessary char types in each group. I think it would be easier to encourage grouping if almost any group was viable. I'd have it so that groups generally needed one warrior as a minimum but that would be the only condition and some priests / wizards could provide that via a pet.
Apart from trying to find and remove disincentives to grouping I'd also want to have incentives and preferably ones that fitted in with the game lore. So I'm thinking of basing most quests / missions around factions with ranks. So say you were lieutenant rank in the dwarf "Ironforge Guards" faction and you got a mission to go bash a dungeon somewhere. Your reward would be assigned something like:
600 exp for success
+200 exp if your group included a player with sergeant rank in your faction
+100 exp for each player in group from your faction or from an allied faction (like the dwarf temple)
+50 exp for mercenaries (any other player that wasn't in your faction or an allied faction)
The dwarf player could try and solo it (and if experienced enough they could maybe succeed) and get 600 exp or they could get a full group of six, all dwarves or players aligned to the dwarf factions, including a sergeant from their own faction and get 1200 exp or get five random players from other factions and get 850 exp. Other players would get a faction based incentive also e.g you'd get a higher exp reward for joining a mission led by a higher rank in your faction but regardless of faction you'd get more exp from group missions than solo ones.