Version 4.3.7284 (beta) has now been released which corrects a lot of the problems listed in this post.
You can download it here: Free/Plus | Pro
Release notes (scroll down for a list of bug fixes).

** Please remember to visit the Google Earth support forum to report bugs and problems **

Discovered so far:

Streaming stops if GE is left idle for a while - need to restart GE or log out and login (File menu)
Clicking a placemark or folder in "My Places" may have unexpected results: the highlight may jump to a completely different location in "My Places" - make sure you are in the right place before deleting anything. Left click before activating the right click menu may be more reliable
Depending on your graphics card, you may not see the sun when using the new day/night function - try switching to DirectX/OpenGL mode (Tools => Options => 3D View tab), make sure you have the atmosphere switched on (View menu), that you're zoomed in and facing East or West, try updating your graphics card drivers. If it still doesn't work ,your graphics card is probably too old to support this functionality
Links in balloons may not open as expected, or may not open at all
If you use SeaMonkey, read this post by smeenz

Problems with 4.3? Please check the support group and the Help Center. If you don't find an answer ask a new question in the support group.

Found a new bug? Please report it in this thread in the support group.

Want to go back to 4.2, but didn't keep a copy? You can find the link under the "Download the previous version" on the GE free and GE pro download pages. If you get redirected to the Google updater page, these are the links to the downloads:
Windows free | Mac free | Linux free
Windows pro | Mac pro
Old versions can also be found on FileHippo. Please be aware however, that this is not an official Google download site, and they will not take responsibility for any products downloaded from there.

KML problems? Check the KML support group

More information about the new features:
User guide
Lat long blogspot

New features:

Navigation:

Overview from Frank Taylor, Google Earth Blog

PS: Beat me to it Ledge - it took me so long to write to this!

Mods: please feel free to add anything to this list that needs adding.