I just set up a new Raspberry Pi and accidentally installed Raspbian Jessie rather that Raspbian Wheezy.
Are there any significant or important differences I should know about?
I'm sure there are lots of differences but none which would make you choose wheezy over jessie.
jessie is the more up to date and will be better supported. You should be using jessie.
A default of
systemd is the biggest change under the hood but, as someone who views this hydra with suspicion, I can reassure you that it is uninstallable from Jessie so that you can go back to (the 30-odd year old spaghetti-monster that is)
sysvinit, the same thing that Wheezy uses as a default should it be an issue for you. 8-P
Extract from the official raspberry web site:
Many of the changes between Wheezy and Jessie are invisible to the end-user. There are modifications to the underlying system to improve performance and flexibility, particularly as regards the control of system processes, and as with any update, there are numerous bug fixes and tweaks. And at the same time as the upgrade to Jessie, we’ve added a bunch of changes and improvements to the desktop user interface.