For me one of the most important aspects of any MMO is finding a guild early on in you characters life. It always improves my knowledge of the game, and will keep me playing the game for longer. I have found without being part of a guild I end up jumping from game to game, never really getting past the mid leveling stages.
While I played Eve Online not being in a corporation (guild) usually meant the destruction of your ship. As without friends (unless you played the forums) you tended not to be aware of politics within an area of low sector space or have the multiple eyes of your friends around the various systems. So at any time you might find yourself at the mercy of pirates or in the middle of a war. Where anything not alliance colours is the enemy.
As a casual War player (about 6 hours a week if I am lucky) I find being part of a guild even more important. I know I will never be part of the guilds inner circle of players or have the best armour. But having a regular set of players to call upon is vital especially in War. Most nights when I login there is already a fully functional alliance war band in the thick of battle which I can just join and sign into Ventrilo and start chopping away. If not and no people are doing city PvE instances or power leveling in Land of the Dead you can quickly get a scenario group together.
You also get to see guild/alliance chat and see questions you did not even know to ask, never mind knowing the answer.
Voice chat also comes in to play in most of the guilds I have been part of. This is also crucial in War as it allows a war band to move as a tight-knit group and turn on a coin. There have been times when the whole war band has hidden behind a rock at a moments notice, or the war band leader has decided to flank the enemy or try a completely new strategy to win the day. When you are part of a guild the leader knows each member of the war band and knows they can rely on each member to follow orders even when it looks like certain defeat. Sometimes it does end in death but more often than not it ends up in fun and a shared unique experience which otherwise the game could not offer.
As well as all the above War also offers special mounts, access to recall scrolls and access to the Viper Pit. So if you not part of a guild, how come?