Good software for watching videos on Mac? I want to watch videos on my Mac but I don't like QuickTime. I was wondering what the community recommends.

In particular, I was interested in some granular ability to slow down the video or speed it up. It seems that QuickTime can only double the speed. I wish I could, for example, say, increase speed to 1.5 or 1.25 (like YouTube). Sometimes I end up just uploading it to YouTube and watching it there - which is, I think, a little dumb.

Also, if it has any additional ability to rotate videos and save them, or concatenate two videos together and save the result; that would definitely be a plus, though not a requirement.
This answer will be short: VLC. Has everything you need except the rotation. It's a media player, not a video editor.

To alter the playback speed in VLC: Go to to Playback → Playback Speed. You can also alter the playback speed using ⌘= and ⌘-.

For rotation, as user Iwillnotexist Idonotexist notes in the comments below, you can go to Window → Video Effects → Geometry → Transform.

Bonus: A list of some VLC keyboard shortcuts for OS X.