I think there are three big things that keep the iPad from totally replacing a home computer for me.
Lesser versions of essential software. For me, this is MS Office. For others it could be video editing software like Premier. The mobile versions of some of the flagship desktop productivity apps are fine for opening and reading docs/spreadsheets/doing light editing, but I absolutely need the full desktop version to do real work.
Games. If you are a gamer nothing can replace a PC.
Can't host a content server. I use plex on my PC to host my music/photos/videos/etc to all the devices in my home.