My first thought was "not that much". But there are some...
-the system menu (NOTE: I'm not sure on this)
-the other channels (shopping channel, weather channel, ...)
-savegames for wii games (of course this only makes a difference if you didn't create a clean nand with modmii).
You could also remove some IOS'es that you don't use for wiiware, but it's hard to say which ones those are (you need a list of all wiiware/VC you want to install and then remove the rest), but I don't think it's wise. For one, it means you may have to reinstall it if you get a new wiiware game, and most IOS'es barely take up any place whatsoever (most are under one megabyte).