Hot Corners Yosemite don't work any more
I had tried this last year but then it didn't work. I tried it again and now it did work:

Start terminal and execute the following commands:


defaults write com.apple.dock mcx-expose-disabled -bool FALSE (this
enables mission control)
killall Dock (kill and automatically restart the Dock, this is
necessary to get it done)


There is an article about hot corners missing dashboard and mission control.