I do a LOT of tab click opening, and I noticed 5.1 made a very subtle change to the behavior. It's annoyed me that when I cmd-click on a tab, it makes a new tab on the far right. When I have several unrelated tabs open already, by the time I'm done opening new tabs via links, they're completely disorganized.
But now, when you cmd-click on a link to open it in a new tab, the new tab is inserted just to the right of the tab you are in. Nice.