A very solid beta. I’ve reported a couple of bugs, but I also have a few observations (mostly UI issues) viz. (those marked with a ~ were, I think, also in 14.10):
Control Centre | Personal | Windows gives “The current window manager is unsupported” when Compiz is switched on; something along the lines of “Use CompizConfig Settings Manager”, or even a redirection to it, would be clearer.
~Default users are set up as a Custom account type, which is a bit opaque. Could they not be set up as Administrator?
~Typo in Applications menu: “pluma Text Editor” -> “Pluma Text Editor”
In Tilda, the configuration window is very wide (the minimal width is fine); also, in Colours | Palette | Built-in schemes, the scheme name is shown twice in the drop-down.
It would be tidier if Tilda and MATE Terminal could use the same colour schemes. This observation is a shameless attempt to get the “solarised” schemes offered by Tilda into MATE Terminal 
Folder Colours isn’t internationalised (right mouse button on the folder pops up “Folder Colors”, and there’s also a “Favorite” amongst the options).
Folders are only coloured in Icons and Compact view, which is a pity as I use List view all the time 
The Bluetooth on/off icon in the Panel doesn’t match the Yuyo theme.
~About MATE states what the version of MATE is; however, there is no one-click way of finding out what version of Ubuntu MATE is being used.
The Ubuntu MATE Rock * wallpapers are an unusual size/aspect ratio (1470x930), which is a pity as they are very good. Could the author be persuaded to redo them as 1920x1080?
Edit: some observations on labels:
(i) System | Preferences labels
The Other category is one of the things they told me not to do in UI school - general “buckets” are frowned on, as they give the impression nobody knows what the contents really are. However, 3 of the 4 options under it easily go into existing items, and the fourth is distinctive enough to have its own viz.
Assistive Technologies goes into a new category, Accessibility
Keyboard Shortcuts goes into Look and Feel
Network Proxy goes into Internet and Network
Software and Updates goes into Personal
I also note that the categories in Control Centre are often not the same as those in System | Preferences. However, they are less obvious so the need to rock the boat is not so pronounced.
(ii) Hover Labels
Mate Tweak and Tilda don’t have them.
That for Plank says “Plank. Stupidly simple” but doesn’t actually say what it does! I suggest “Launch your applications easily”
That for VLC says “Read, capture, broadcast your multimedia streams”, which sounds a bit techie (to use a horrible word). “Play videos” is what 98% of the users will be doing.
