Don't Install if You Use a Really Old Mac
The macOS Mojave update isn't coming to all macs that could run macOS High Sierra, but it is still coming to many older Macs.
If you use one of these old devices, you may want to wait to see how macOS Mojave runs on it for others.
This will let you decide if the performance is fine, or if there are issues that will prevent you from reliably using your Mac.
You'll be able to downgrade to macOS High Sierra, so you can test it out, but you'll want to make sure you have a good backup and that you have time to downgrade if macOS Mojave isn't what you expected.