Gotta say I love the addons--they're endless, and if you ever get bored of one thing, you can just swap it out for another thing.
Addons for everything, everwhere--all sorts of new effects files, modifications, old and new planes, helicopters, tons of ships and other such items, plus all the scenery...
But I think I like Quick Combat the most--especially when you can grab the QCEditor and face off against your addon aircraft.
The Mission Editor is awesome too--can't get enough of it. I haven't uploaded anything as of yet, but sometime soon I might start doing just that. A few payload modifications to aircraft, some missions, packs... Who knows! Heheheh.
Best of all--the online community that goes along with CFS2. All sorts of people out there--in here... Many grand people.