- QuickView doesn't work properly
- The whole OS is very laggy if you have more than one app open (and this is on a one year old MBP)
- Finder is very laggy always
- iMessage keeps trying to use my old account rather than the new one I set up iCloud with
- My laptop doesn't go to sleep properly when plugged into an external monitor (although this bug was present in previous versions too)
There probably more I can't think of right now, but this OS is very frustrating, especially when it slows my computer down to a crawl. I can only hope Yosemite is an improvement.
Not to discount what you're saying, but I'm running 10.9.3 and have no issues with QuicKView, general OS lag, the Finder or iMessage. Can't speak to the external display bug you mention as I don't use one, but regardless, I'm not experiencing any of the issues you are and I'm using a 2009 MacBook Pro. I genuinely recommend backing up, wiping your Mac and then setting it up from scratch. It sounds like a pain, but the only explanation for the issues you're having is some combination of either a botched upgrade or the like.
I'm running a 2008 MBP and it runs perfectly with Mavericks, and the 2012 iMac is tickety-boo also. I did what J@ffa suggests, with a clean install though, to get things started off. Try logging out and then back in to the icloud site, that sorted things for me, when Mavericks was first released and I had message problems like yours, 0dev.