You can use git to generate a list of changed files (and individual changes) in 1.8 (or between any versions really).
Grab the mybb repo, checkout the latest 1.6 tag and generate a diff against the 1.8 tag.
/tmp git clone https://github.com/mybb/mybb.git
Cloning into 'mybb'...
[...]
/tmp cd mybb
/tmp/mybb feature git checkout mybb_1615
Note: checking out 'mybb_1615'.
[...]
/tmp/mybb ➦ c243e6a git diff --name-only mybb_1800
[..list of changed files..]
/tmp/mybb ➦ c243e6a git diff mybb_1800
[..giant list of all changes..]
As others have said, pretty much everything changed so there isn't a whole lot of value in it this time, but its a bit of FYI for the future.