"Tag '...' is too long"
I eventually tracked the problematic tag down to a document I had recently added. Seemingly, Zotero had extracted huge chunks of the abstract as separate tags by accident, so I deleted the tags, and tried the sync again. No luck... it still complained. Then I remembered that I had accidentally stored the same item twice, and deleted one, so I purged the trash can... but still no luck. Sync was still complaining, even though the tag no longer existed.
A quick search of the support forums for Zotero 1.5 Beta showed a few people with similar problems, and a little digging uncovered the solution. Hopefully Dan won't mind if I quote the relevant text here:
type about:config in the Firefox address bar, hit Enter, and search for the extensions.zotero.purge.tags pref. Set that to true by double-clicking, and then close and open the Zotero pane. The unused tag should be deleted, and the sync should proceed.
That worked a treat. Thanks Dan! :D