1. The most sluggish start up ever
The one thing which made OSX so superb in my opinion was the lightening fast start ups, once the machine was turned on, you were good to go. You could do anything, immediately. Now, the total opposite is true. When the OS is loaded, it is immensely slow for several minutes until the bowels catch up with the visuals.
Upon start up, once the dock is loaded, if one clicks an application, it will happily bounce there for what seems like an eternity. Almost nothing responds instantaneously anymore. When clicking an application, it will bounce and bounce and bounce. Right click the app to quit.... the dock will freeze for a couple of seconds and you will have to force close or just wait for the bouncing to stop. Open up multiple apps.. well you might as well go and make a coffee while you wait.
This is perhaps the most irritating thing about the update; the general sluggishness on start up. I should mention that I am not only talking about a boot up from shut down, these problems occur when awaking from sleep. Very rarely, this will not occur or it will be short lived, it literally occurs every time.