As another poster in this thread mentioned: bookmarks and other personal data syncing. Allowing 3rd party sync servers would go a long way towards Chrome projecting the message they don't want to lock you in but that's probably not the case -- if we judge by their team's [lack of] actions.
(And don't get me started on services like XMarks. They're miles behind.)
I won't directly mention the project that's best placed to do that, as it's not yet ready for primetime, but let's just say Chrome may not have the performance crown in the next 5 years.
Also, aside from on the Mac, what's holding back Firefox? I accept that the Mac version seems to have more problems than the Windows or Linux versions (I have no technical explanation for this, it's just what I've heard anecdotally), but on Windows it seems to be a solid choice. Is it just about momentum for Chrome rather than superiority at this point?