With the addition of the new launcher Dinnerbone is working on and the API coming on it's way soon, an idea sprung in my head around the multiple .jar support.
1) Modders would be able to publish their API compatible mods (or mod packs) to Mojang.
2) Once approved, they could appear on a master list of mods for people to browse and easily download separate from vanilla Minecraft.
3) They should be automatically sorted by singpleplayer and multiplayer capable mods, because some people will have issues getting them to work both ways.
4) By default, the most popular mods (most downloaded/played/highest rated/etc.) would appear on top of the list.
5) Some newer mods would appear on a "modcast" if you will, where people can download and play them, possibly giving the modder some ideas for future version.
Yes, this is entirely like how Blizzard handles community maps on StarCraft, but that's kinda the idea. The system works, and I have no doubt this would be successful too.