The 1.5.1 ChangeLog lists bugs that have already been fixed in the subversion repository. The ChangeLog is updated as bugs are fixed. These fixes will be present in the 1.5.1 release (no date yet, don't ask :-) Check out the ChangeLog to see if that bug that has been annoying you in 1.5 has been addressed.
You can also follow 1.5.1 fixes via the changeset timeline. Every fix committed to the repository will be listed here in reverse chronological order. If you are so inclined, you can click on a changeset to see exactly what was changed.