What are the most significant differences between how a Linux bridge and an Open vSwitch work?
Open vSwitch has a section on that in their FAQ:
Q: Why would I use Open vSwitch instead of the Linux bridge?
A: Open vSwitch is specially designed to make it easier to manage VM network configuration and monitor state spread across many physical hosts in dynamic virtualized environments. Refer to Why Open vSwitch? for a more detailed description of how Open vSwitch relates to the Linux Bridge.
The entire list of features/differences (Why Open vSwitch?) is quite long, you should check it out if you want to know all the details. The short version is that Open vSwitch is targeted at large multi-server virtualisation environments, so it is focused on logical abstraction and management; the Linux bridge is fast and reliable, but lacks all fancy control features.