Hotkeys, there's some nice ones for Mac.
Command + Option + D to open and hide the dock, much better than auto-hide or just leaving it up all the time and taking your window space.
Command + Alt + Escape opens the force kill window if a program is sticking (similar to CTRL + ALT + DEL on Windows)
Apple->About this Mac->Storage for a quick overview of your hard drive usage
Launchpad->Other->Activity Monitor - similar to task manager in Windows but more useful because it makes more sense. Memory Pressure field tells you how hard your RAM is being hit
Apps installed from the App Store are super easy to uninstall because you basically just delete the icon from the Applications folder. Web programs (stuff you download from places besides the App Store) are a bit trickier and sometimes need an uninstaller that ships with the app to clear everything out
Click anywhere on your desktop to get the Finder menu. Finder->Preferences lets you display a folder for your hard drive right on the desktop (similar to My Computer on Windows) lets you easily get into and browse your files. Basically works like old school Mac OS from the 80s/90s so everyone should use their Mac this way
Mouse: Well I definitely like Apple's touchpad. They have some of the best gestures. Three-finger swipe basically functions as holding the mouse button down and works really well. The Magic Mouse is only ok - it has some really cool touch gestures but does not have sensitive enough mouse movement and does not always read clicks correctly (it uses a single button like older Mac mice). Personally for gaming a recommend a Windows two+ button mouse. I use a basic Logitech one but any USB or bluetooth mouse is going to work just fine.