Awww I should have totally said something about being trapped for the past two years haha.
The lack of a job/class system is something that has been used in many mmo's to great success. Two fantastic examples are EVE online and Star Wars Galaxies. In both games, the player has the ability to choose what they want to level up and when.
In EVE online you have the choices of choosing to level up a skill such as a mining skill, crafting skill, or combat skill. It is very similar to SAO as SAO had some same general principles but restricted the player to a maximum amount of skills they could actively hold.
Most players in SAO chose to focus primarily on combat skills, such as toughness and fast regeneration, or single-handed sword and shields. In SAO, everyone started with two slots and then recieved: a slot at level 6, a slot at level twelve, and a slot at level 20, and then a slot every 10 levels after that. When that player placed a skill in a slot it could then be leveled up. But if you removed a skill from the slot to replace it with another skill, that skill would lose all of its progress and if you wanted to use it again you would have to start over.
This is the absolutely perfect system for making a dynamic community. You get some players who want to have nothing but combat skills, and others that want to focus on crafting or trading. It allowed Agil to fight on the front lines while still letting him be a trader. It allowed Liz the ability to go collect minerals while still being a weapon-smith. But the only reason they were able to fill multiple roles was because their levels were so high. For the average player (keep in mind I believe it was 5000 individuals who never even left the town of beginning), those two skill slots were only enough to do a single job.
In essence, real communities of traders and fighters were formed allowing for diversity and growth. Much like the separation of levels (front-line, mid level / bandits, low level hunters, permabeginners) had their communities, so would the trades.
On a fun note, This is why Kirito is so surprised when Asuna says she maxed out her cooking level. I believe at that point she had 8 skill slots, so using one on the cooking skill (a pure luxury) as opposed to something that could possibly save her life was pretty crazy.
I'm not sure if that helped explain anything, I love to answer questions about game design though. I was just at the game developers convention this year focusing on design workshops, and it really amazed me how much thought Reki Kawahara put into the overall design.