My main issue, having used the RTM for a few days now is that they haven't entirely unified the UI yet. In some places you still get "old windows" instead of the new Metro-inspired interface. For instance, the "Settings" app is supposed to have entirely replaced the old "Control Panel", but the settings app does'nt have all the same functionality, so if you do some things it will revert to the old control panel.
If the rumors are true, the UI being switched from old Win32 to XAML is an ongoing process that'll be complete by the time the second Redstone update rolls out next year. Right now, everything's in place for the future upgrade, but it hasn't been fully committed to as of yet, probably due both to a lack of time, and to maintain compatibility while Win10 builds its user base.