1. Depends. Better if the dock supports PnP, better connectivity such as the eSATA or the newer USB 3.0 etc, better cooling since it's left open, support for drives with larger capacities etc.
One thing you need to be careful about these docks is the heat issues. These docks are not meant to be used continuously. These are better used as backup devices where you backup your data once in a while or at the end of the day. If you want to use these docks continuously, better look for a HDD enclosure with a fan instead.
^^ Not really. The hard drives are out in the open, so I don't see any reason for overheating issues with docks. That seems more an issue when you use drives with those casings, even though they have fans.
i have mascool dock, never faced any issues of overheating till now, sometimes i have it running continuously for over 3 to 4 hours when i am watching movies.