So there I was, in the Silithus desert...
I want to talk about solo-friendly MMO gaming. What is it? Why? How? Marshmallows.
Why would anyone want a solo-friendly MMO?
I see two reasons. First, it keeps scheduling flexibility. If I log on before my friends and guildies (but not so long before that we're in "get a different guild" territory) I won't want to sit around bored. Give me something to do while I wait for my friends.
Second, a person may simply desire to be alone at the moment. Maybe they want to kill some foozles without needing someone around. They're just not feeling social. This may seem like a strange situation to accommodate for a game type based on multi-player play, but everyone has their lonely moods and it is potentially profitable to be able to keep people playing in your (dev/comapny's) world, rather than going somewhere else and learning something awful like "they don't need you."
How do we make a solo-friendly MMO?
I see two general methods. One is the obvious one: make quests and content easy enough to be done alone by most or all classes and specs. Most gathering and crafting fits this model, with relatively few recipes needing materials that don't come from the corresponding gathering profession. Except engineering. Most quests also fit this model. Note that this model fits will with either reason for soloing: unavailability of friends and desire to be alone.
There is the opposite approach as well: make groups easy enough and quick enough to find using in-game tools and social aspects that a player does not need to be with their guild to participate in group content. This can come in two forms: tools and location.
Tools are things like LFG channel (I miss thee), the early LFG tool (LFG three heroics, plus all the other ones that I cannot select because apparently no one actually tested this for anything but bugs), and the current LFD tool (LFG anything, just don't talk to me). Oh and the elite quest grouping tool. Lol. Or was that part of the LFG tool that no one used for anything beside instances? These served the function of putting people with common goals in contact with each other when they might not have otherwise (especially cross-server). They're really great at what they do.
Location means putting them in the same place. If I'm struggling to kill elite bugs here and you're struggling to kill elite bugs there, maybe we could work together and not struggle. What a crazy idea. This works well when we're both in Silithus. If I'm in Silithis and you're in Blasted Lands, not so much. The idea here is that players will naturally congregate in certain areas for similar goals and the challenges in those areas will be such that they can play alone, but will notice a significant benefit from working together. You might have noticed that I am using Silithus as an example, probably because it's a good one: hives filled with elite bugs and multiple boss-level mobs that could be summoned, one type even requiring a small raid to beat. Note that this has changed a lot since then: at 85 you can solo all of it and the bugs are no longer elites anyway.
Solo-friendly does not have to mean trivial
Like the bold words say: solo-friendly does not have to mean trivial. The previously mentioned bugs were not easy to solo. They could be, so you don't need friends, or anyone at all, to quest there. For me they're a convincing piece of evidence for the location method of solo-friendly play. You can play alone. You can play on a random schedule and play with whoever you happen to run into, which you are likely to do because a meaningful, useful location is going to be a popular spot.
In other words, a game can be solo-friendly without discouraging grouping or encouraging sociopathic behavior. However rigid quest structures and the anonymous, reward-driven LFD system, each do at least one of those.