D:\World of Warcraft\Data\Cache
Will anything extraordinarily bad happen like the game not running and such?
I just thought about it because its 7.81GB of cache files and do I need them or can I delete its contents without resulting in a problem same way as D:\World of Warcraft\Cache?
Yes, you can delete it, but after you do so, your loading screens will be much longer (with the small progress bar below the big one), since the game will again repopulate the cache. So it won't work as a space saver.
E.g., if you want to copy your WoW folder to another computer fast, you can skip copying the ~8Gb data/cache folder.
I don't know details of this new caching mechanism, but I can't quite figure out how much performance you can gain if you cache ~8GB worth of data from the ~16GB normal data files.