Well, it is not only about multiplayer. With the FOnline engine, you can do pretty much whatever you want, unlike the Fo2 engine, which is hardcodet from a to z. It took years to get access to even just simple stuff... and this only via hooks and stuff into the engine with Sfall. So it's all nothing but some tricky hackery.
Simply the huge maps you can build in the FOnline engine just aren't possible in Fo1/2. Also editing the graphics - FOnline engine supports all kind of graphic formats, while Fo1/2 only eat converted frms with a limited set of colors, editing the sound, etc. etc. it is all so much easier and more convenient. Another example is the small terrain tiles. You don't have to use them at all. Nothing hinders you to just create a huge scene in a 3d programm, render it to 2d and put it into the game as is.