I spent some.. Oh, 11 months without a guild. I completed prophecies few times, there were month or two breaks away from the game, been around ever since Factions release again, and after completing the Factions storyline few times I had it.. I either join a guild, or permanently quit the game.
Soo, I looked around more or less actively, tried GWG's looking for guild thread, joined one, the guild died next week, ohwell. Then I was just hanging around in Cavalon, when there was a bit different looking 'guild looking for members'-spam. I made a joke about the guild's name. The recruiter replied in a civil manner, as jokingly. I replied with another joke until I said okay, invite me in.
Been enjoying being in a guild ever since that. Buddies to do AvA and other stuff with, occasional help if I need any, and some helping to do when bored. Some fun peeps to chat with while wading through boring quests/missions etc.
So yes, you can play the game without a guild. Being in a good, nice guild adds much, much more fun to the game, so it's really recommended.