My two biggest complaints about the Xbox One are it's achievements and it's seemingly inability to update games unless I launch them.
Xbox One can be always online so updates are a thing of the past however every time I start a game up, I need to start an update. This should be done in the background without me needing to prompt it for all installed games.
Definitely backwards compatibility with the Xbox 360 games, that would be the very best feature they could have. However it's not likely to happen which really sucks...
One thing for me is definitely not having backwards compatibility. The 360 had it even though not all games are supported but at least it's there. I know having BC will probably increase costs and price so it's better to not having it at all. I guess it's not too big of a deal as I can still play them on my 360.