Since we're talking about diff features, I thought I'd throw out some that people might not know about. Some of these only SlickEdit has because they use SlickEdit's context tagging engine.
Maybe you could add "diff a range of lines" to this list. I use the xcompare_diff_region I posted here quite a lot. I would be happy if you added these to slickedit. I know you can do it through the diff dialog but it's quite laborious.
https://community.slickedit.com/index.php/topic,11186.msg47177.html#msg47177"History diff" needs a "save" button. If you change the code directly in diffzilla, there's no indication that you changed anything and the only way to do a save is Ctrl-S or exit the diff and use the exit prompt.
Well, I noticed some problems in beta3 while writing this so here they are.
1) Diff the current file with a version from backup history, make a change to the file inside diffzilla, then use Ctrl-Z repeatedly - the result is a bit weird.
2) Make a change to the current file without saving it, then diff two files, one of which is the file you just changed - and using "compare symbols all". The buffer you had open loses the changes you made and gets some green highlighting in the places you had changed. Now do it again but this time, save the buffer before you run the diff. Diffzilla says "no differences" and the buffer somehow loses the changes you just made and saved - it does on my machine anyway.
3) When you diff two files and select "compare symbols all" for path one, for path two you can't select "compare symbols all". I'm not sure why path two even has a dropdown list for this ??.
4) Diff two files on disk one of which is open, diffzilla pro opens with the file on the right (the one that is open) marked as read only (why?). When you right click in the right hand pane and select "edit file", slick ignores you and keeps diffzilla open.