I personally like OS 10.10 more than OS 10.9, however, as you can see from this discussion thread (152 pages long!!) on Apple’s Support Forums Wifi issues continue to persist.
The common thread that I deciphered from all of those complaints is there is a Bluetooth / WiFi incompatibility is. You all know what WiFi is (wireless internet). You use this if you are not directly plugged in to your modem or router. Bluetooth is another wireless technology. It allows you to connect to wireless mice and keyboards.
Even if you don’t use one of those input devices, your Mac may have Bluetooth turned on by default.
Apple menu (top left of screen) >> System Preferences >> Bluetooth >> Turn off. If you are using keyboards / mice with your Mac, make sure they are wired. Apple will fix this problem eventually but right now the signals are crossed, no pun intended.