It took me almost two months to discover the tab "Change history" on the issue details page.

I thought that changes like a change of priority etc. simply aren't logged.
In fact they are but the log is buried behind the "Change history" link.

I think that it would be better to combine comments and some of the most important "Change history" elements on the default tab.
I would suggest displaying a long, chronological list of comments, changes (readable format) etc by default with a kind of filter section on top of it, like:
[ v ] show comments [ v ] show changes etc.

Table like "original value | new value" isn't readable enough.