There's a significant difference between WFC and XBL, even beyond the convenience of punching in Friend Codes.
XBL was designed with the idea of always being connected, even when you aren't playing online. You can check your friends list—which is universal, of course—and find out what all of your friends are doing at any given moment. Be that playing an online game, playing an offline game, or watching a DVD. You can invite anyone who's online to come join you for a game, even if they're not playing the same game. (They'll change discs to accept the invite, of course) You can also check your friends' progress in their games. That's mostly trivial, but one more unique advantage to the XBL approach. One other cool feature is the notion of one-on-one chat—you can chat with a friend one-on-one, no matter which games either of you are playing. This means that you could both play offline games while still chatting. Or different online games. Or whatever.
Even beyond the concept of friends, XBL has advantages. With Friend Codes, you have to specifically add anyone you wish to play with. That works fine for your existing friends, just with more work, as David said. The advantage XBL has is matchmaking. Say that, In Super Smash Brothers: Brawl, you just wanted to spend an evening playing online. With the Wii, you'd have to go on MSN and hunt down specific friends to play a match with. They may not be online, or may only want to play a few matches. But with matchmaking, you can play against random groups of people for as long as you'd like. While I'm looking forward to playing with everyone in Brawl, I'd love to be able to practice with random people when no one else is online. You can make new friends that way, too.
Ultimately, WFC is pretty disappointing from a multiplayer perspective. It's functional, but just barely. XBL is so much more robust and convenient. Now don't get me wrong—I don't like Microsoft or the 360 by a long shot. In fact, one of the disadvantages of online gaming is that it attracts an adult group, so you have to be willing to tolerate a lot to play. The only reason I'm willing to draw comparison is that I want WFC to be better. I think we all hoped that the online competition from last generation would motivate Nintendo to make a great online service. Now I guess we have to hope that the Wii's successor can implement better online, while still protecting that family-friendly image. I'd imagine that's really hard to do.
