what you 'can delete' depends on exactly what you're currently using/wanna use on the wii.
*You should always just move files to your PC - so you can reinstate them if there's any unexpected problems*
but at a glance I would offer:
.bin files
nand.bin &keys.bin
are important - keep these stored somewhere for if you need them
nincfg.bin - this is the configuration settings for Nintendont (Gamecube games player)
(x6) .wad files on root - these normally can be removed once installed. You'll probably have a bunch of them in 'wad' folder as well.
(x6) text docs - not sure about 'APACHE-2.0' or 'installer' - but you can remove the other four
syscheck - can be removed
FOLDERS:
wad - usually holds .wad files for install, once done not needed until you're gonna uninstall em
vbagx - holds support files for Gameboy emulator
fceugx - holds support files for NES emulator
txtcodes AND codes - normally for game cheats (.gct)
bslug - files for brainslug (online play) I've never used/installed it so I dont know?
Riivolution - normally contains some .xml files for disc patching on the fly with Riivolution app
NeoGamma - holds support settings file for Neogamma app.
Private - think thats for titles you have saved to SD within the wii settings
apps -
leave that one there! its all the apps you have in your Homebrew Channel
hope the above helps a little