This update is the biggest Homebrew stopping update till now. Some effects on Homebrew are (green = solutions):
Deletes the Homebrew Channel ► Homebrew channel got new titleID (JODI instead of HAXX)
Blocks Bannerbomb, other exploits remain unchanged (Twilight Hack was already blocked on 4.0). ► New version of bannerbomb was released
Overwrites boot2v2 and boot2v3, overwriting BootMii on consoles without boot2v4 already installed. ► Reinstall Bootmii after updating (if you could install it before)
System menu now uses IOS70, as attempt to block preloader ► Preloader can use IOS70 now
Known cIOS (249, 250, 222 and 223) get overwritten by stubs (unused placefillers) ► Delete them with Wad manager (or AnyTitle Deleter). IOS250 doesn't get overwritten if you already had it.
You might want to know if there are any advantages. Well, the answer is MAYBE. This update was only to block homebrew, but many IOS versions were also updated, so that could possibly show better performance while using your Wii.
But still, if you want to start fresh, I'd recommend updating to 4.2 as its not too different to softmod it.
WARNING
Do not update to 4.2 if you have a region changed Korean Wii. If you do, you get the famous 003 error (=brick). If you really need the update, install IOS70 from cIOSCorp 3.5
More info at: http://wiibrew.org/wiki/System_Menu_4.2