"I would prefer that you didn't submit this" is too personal and negative.
One of the goals of the switch to Git and Gerrit is to make MediaWiki development more welcoming to newbies by making sending patches easier. This message does exactly the opposite - it essentially tells the person who wrote the patch: "Your work is bad, you wasted your time and my time and I don't like you".
A better wording would be something like this: "This commit must be fixed before merging". Even SVN's "FIXME" is much better than "I would prefer that you didn't submit this", because it's immediately clear that it's a technical and not a personal thing.
I also commented about this in the Gerrit bug tracker [1], but if it's possible to fix it locally, it should be done ASAP.
[1] http://code.google.com/p/gerrit/issues/detail?id=1241
Version: unspecified
Severity: major