In a way, wad2iso is even more a triiforce front-end than mighty channels (or triiforce mrc). wad2iso's forwarders do nothing but load up triiforce and then launch the game with predefined settings. It's a bit like a shortcut (or rather: a batch file) on a computer.
@OP: there is no way to run wads directly. They always have to be installed on the wii's nand...or on an emulated wii nand. After setting up that nand on your usb drive, you can use showmiiwads to install wads to that nand. After that, it's a matter of getting the right program.
-Triiforce and triiforce mrc are the most common ways. mighty_channels is the successor of triiforce mrc. Although it is currently still in beta, I'd say it is stable enough to run most (if not all) games that triiforce (mrc) can play
-wad2iso is a very convenient way to play wiiware if you're using an usb loader. keep in mind, though, that it is basically triiforce you're using (in other words: if you can't get a game working through triiforce, there is no way it'll work with the created forwarder).
-uloader can also play wiiware and VC from an emulated nand. It does not rely on Waninkoko's rev's, and as such, it can play some wiiware the previously mentioned methods can't handle. The interface isn't great, though, and compatibility isn't 100% either.
-sneek (or rather: uneek) is a way to completely emulate the wii on SD- or USB drive. It is harder (more complex) to set up than the earlier methods, but if you can manage to get it working, it has 100% compatability.