Today I upgraded to 2.3.4 Gingerbread MR2 but I wonder: what changed?
Similar to this question which asks about 2.2.1, there don't seem to be any real "changelogs" made available for Android updates. However, you can look at their git repository to see the commit log. Here's the shortlog for the 2.3.4_r1 tag.
The summary: mostly bugfixes, and the addition of video/voice chat for the Google Talk app. Apparently this is only enabled on the Nexus S since the Nexus One doesn't have a front-facing camera, but I don't know that for certain.
In any case, the video/voice chat could probably be made to work on the Nexus One somehow considering that it has already landed on the CyanogenMod codebase (and the latest nightly builds), so it's more or less working on other "unsupported" phones already. I know the EVO has a working implementation in the latest CM7 nightly build, though it did crash when I tried to switch to my rear camera (not that I'd really need to do that).