I started late in the beta cycle. I performed fresh install in both cases
1) I found that application windows such as maximized Thunderbird did not display completely - by that I mean the min-max-close controls were not always visible. Since I really didn't like Unity anyway I choose to install Gnome-flashback which provided the correct display of the controls. Also it was an easy fix to move the controls to the upper right.Overall I give 14.04 an A minus since #1 doesn't matter to me and #2 didn't affect my desktop HP
2) I tried to make the top panel transparent on my Dell laptop which caused 'Indicator Applet Complete' to fail upon boot. I attributed this to a relatively slow cpu / hard disk; the remedy was reverting to 'use system theme'