The difference between Singleplayer and Multiplayer can be huge.
It depends on how you look at it.
If you enjoy playing by yourself against NPC's using the same AI all the time except for some MOD's that crank up the NPC's skills and some that also let the NPC's use Nano bots and Shield batteries.

Multiplayer is what you make of it. Some characters enjoy playing against real pilots thank think on their own. Also some of the online servers have communitys that you can join and meet some new people and even make new friends and some even have websites, teamspeak and or ventrillo servers, IRC, etc..

Teamspeak and Ventrillo are overlooked.
They offer free voice communications, think of a cell phone where you can talk to anyone in the world that has the software and you're on the same server.
Great for talking to people that are playing the same game you are or even talking to your girlfriend or grandmother but no long distance changes as long as you have a computer and internet connection, which you're paying for already so why not use teamspeak or ventrillo and talk for free...

I wish I had more time to play, any games for that matter.
Well that's my opinion anyway.
Although "when I was younger I could remember anything...
Weather it was true or not"...

For me, sp is fun

but MP has far more opportunities, interaction with other players
is a lot less predictable, and the game feels more immersive

I post some interesting reasons to play FL MP here.

As for other reasons bp and elle have said most of it. The one thing I think is the most important difference is with playing the 'puter, once you beat it you usually continue beating it. You figure how to win against it. Against other players with their myriad of styles you will never get tired of the endless ways you go boom or make others go boom.

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Earendil
SysAdmin of Boston Freelancer server
Server community website & forum: EarendilsPlace.net (server rules)

Roleplay also works better with other people to interact with