Test a project by clicking in the timeline between two clips and press COMMAND and T, see if a cross dissolve is applied. This is the default shortcut for a dissolve (unless you have modified the shortcut). If it works, then relaunch FCPX while pressing OPTION-COMMAND keys and delete ALL preferences. This may restore your transition thumbnails, especially if you never did this when updating FCPX. It's recommended to do at first launch of every FCPX update.
The COMMAND T adds 15 fames of black, and holding OPTION-COMMAND and deleting preferences didn't work, though interestingly on start-up, it wanted to show me "all-new features," as if there had been an update. Any other ideas? I really appreciate the help. It's INSANE that these two features are missing.
The "New Features screen" is normal to see when you startup the first time after deleting preferences. Did you try Ben's suggestion? Also, did you move any folders or plugins out of the default Users/Movies/Motion Templates folder?
Also, go to the Final Cut Pro menu (Apple technically refers to that as the "Application Menu", whatever), from there select Commands. Be sure Default is checked, even if you have your own custom set, just to test from.
After you do that, go to the Apple menu, System Preferences, Keyboard, Shortcuts, Keyboard, "Restore Defaults".