Biggest may be that a lot of labels lose their connections when upgrading, and turn into a "broken" label that includes a GUID instead of the connection to the correct field. There was also a change in C3D 2011 that caused some Parcel labels to appear at the wrong offset from the Parcel line, vs. in C3D 2010 and earlier. And if you've been creating custom Reports or Subassemblies, you may have a lot of problems with those, as well.
For the most part, except for the fact that some of your Styles may "lose" their connections and turn into GUIDs, as mentioned in my last post.
Also, there have been some new features added, which involve new Styles. So if you simply migrate your 2010 template to 2013, you may find that you have to manually create some Styles for some of the new features (or copy them into your template from one of the stock C3D templates).
The way section views are organized was significantly changed from 2010 to 2011. Opening a 2010 file with XS views will probably result in them moving, especially if you've moved some of the views manually.
Cansel - Autodesk Division
I think it means "Globally-Unique Identifier". It's a VERY long string of letters and numbers, which is typically used in Microsoft Windows-based software. It's created in such a way that it will ALWAYS be unique, no matter how many people try to create them. You'll see a whole bunch of them if you go into REGEDIT and poke around... Virtually everything in Windows has a GUID.
The format is something like this (and here's one I JUST generated...):
When your C3D labels get corrupted, which seems to happen in upgrades from C3D 2010 (and earlier) to later versions of C3D, you'll see your label values replaced with strings like that.
Oh, and I suppose it should go without mentioning that if you poke around in REGEDIT, be berry berry careful... Der's wabbits in there....
(In other words, you can SERIOUSLY mess up your system if you do the wrong thing.)
To find all the new styles or those that changes for each release, 2011, 2012, and 2013, you can go to the online help and under the new features they list all the new styles and and changes. This helped me to migrate my 2010 template to 2012 then to 2013. The list is usually at the bottom of the new features page.
Civil 3D 2013