I gave about an hour ago a one-line answer to this question, stating that a finitely generated algebra over a field is noetherian, and that a previous answer (by me, among others) had shown that the algebra was not noetherian.
My answer was removed and replaced by a comment, under which stood the (unsigned) explanation that the answer was trivial and hence converted to a comment.
That the answer was trivial is relative: it tautologically wasn't for the OP but it is certainly also true that anyone with a moderate command of commutative algebra will find it obvious.
Anyway, my rationale is to keep answers and comments disjoint: if someone asked how much is $2+2$, I would answer $4$ in an answer box and not in a comment. Call me literal-minded if you wish...

Edit
I thought the conversion to a comment had been done by a moderator, but Martin's comment below and Mad Scientist's answer explain that it is actually an automated process.
I apologize to all, but especially to the moderators, for this misunderstanding on my part.

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This is an automated process, executed if the software decides that the answer contains too little information besides a link to another answer. See: What constitutes a trivial answer? and Answer appeared automatically converted to a comment. –  Martin Jan 10 '13 at 9:33
Ah, I see, thanks a lot for your answer Martin: +1. The software "decision" was wrong: my answer did not consist of a link: the link was there to explain why my answer applied! Anyway, I will delete my question. –  Georges Elencwajg Jan 10 '13 at 9:39
Oops, now the software doesn't allow me to delete my question, because there is an answer to it! I have some strong opinions about this software but I won't be able to formulate them in decent language... –  Georges Elencwajg Jan 10 '13 at 9:45
Automated "low quality" check, "short answer" redirection, automated reviewer audits... StackExchange is becoming a virtual city patrolled by an army of RoboCops with limited artificial vision and a handful of bugs in their software. –  user53153 Jan 10 '13 at 21:46

No human was involved in this process, as already mentioned by Martin in a comment, this is an automated process.

The important criteria are length and the presence of a link to another SE post. Simply writing a short answer won't trigger the conversion.

If a question can be correctly answered by a few words and a link to another answer, it is likely a duplicate anyway. The SE software and the general SE policies tend to discourage very short answers and strongly encourage users to elaborate on their answers and explain them in some detail.

I'm speaking about the general feature here, not this specific case as I can't see the deleted content anyway.

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Thanks, Mad (but wise!) Scientist, +1. –  Georges Elencwajg Jan 10 '13 at 9:41
Nice to know that the software recommends incomplete referencing. –  Michael Greinecker Jan 10 '13 at 13:08
Tinyurl or similar would be a workaround... –  user1729 Jan 10 '13 at 13:33
@MichaelGreinecker The auto-conversion is only triggered on very short answers. If you write more than one sentence, you are usually safe. –  Mad Scientist Jan 10 '13 at 14:21