Q:

Break from conventionality in Mahler Symphony no. 1

In the Wikipedia article for Gustav Mahler's Symphony No. 1, in describing its structure, a paragraph ends with

The usage of F minor for the last movement was a dramatic break from
  conventional usage.

But I'm unsure what the author of that article meant:

Was the use of F minor in particular unusual, or
Was the use of the most musically distant key from the Symphony's D major tonality a "dramatic break" from conventionality?

And, assuming either, what was unusual about this?
For the record, I am a big fan of Mahler and his music, and I'm also aware that he was constantly "pushing the envelope" with respect to progressive tonality in his symphonies.  But, being his first, and considering when it was written (1887-1888), then perhaps this sticks out, even in the context of his works, as unusual.
Also, I'm aware that this is Wikipedia, and one should take whatever's published there with a grain of salt.  (Not to mention there's no specific citation for the above quoted sentence.)
But still, even I think that how the finale of Mahler 1 starts is quite unusual in and of itself--it's as if the composer lulls the audience into complacency in the 3rd movement and then tells everyone to "wake up and pay attention!" in the finale. :-)

A:

The standard convention for a symphony was that the first and last movements were in the same key, or occasionally the major and minor with the same tonic (e.g. Beethoven 5). 
Calling this a "dramatic break" with convention might mean it has a dramatic effect, or it might just be purple prose for "this is a bit different from normal". Note that the symphony as a whole does end in D major.
Beethoven used key schemes based on intervals of thirds - e.g. the Sonata Op 106 first movement has sections in 
Bb major -> G major -> Eb major -> Cb major (written as B major) and then a semitone drop back to Bb major.
Schubert probably got the same idea in his own works from his idolization of Beethoven.
F minor -> D major is certainly within the vocabulary of late Beethoven IMO. 

A:

A Symphony conventionally should have four movements in a structure of either opening - slow - dance - finale or opening - dance - slow - finale.  Mahler chooses to write in opening - dance - slow - finale structure:

Gustav Mahler: Symphony No. 1 in D major

I. Introduction and Allegro comodo. (D major)

II. Scherzo. (A major)

III. A la pompes funèbres. (D minor)

IV. Finale: Allegro furioso. (F minor - D major)

Mahler chooses the key of D major for the first movement, A major for the second, D minor for the third, and F minor for the fourth.  As Wikipedia says, the usage of F minor for the fourth movement was a dramatic break from conventional usage.  The opening and finale movements of a symphony should be written in the original key.  In this view, the finale of Mahler's Symphony No. 1, in D major, should have been written in D major, not F minor.  However, it ends in D major, which is the original key.

Beethoven also had did the similar thing before:

Ludwig Van Beethoven: Symphony No. 5 in C minor

I. Allegro con brio. (C minor)
II. Andante con moto. (Ab major)
III. Scherzo: Allegro. (C minor)
IV. Finale: Allegro. (C major)

Beethoven does not choose the key of C minor for the finale, but rather C major.  The triumphant and victorious mood greatly contrasts with the first and third movements, which are dark and emotionally stormy.  Unlike Mahler, Beethoven never returns to the original key of C minor and is not even willing to do so - therefore, the key of C minor is never to be heard again at all.

In order to see a much more extreme case of such a rule violation, here it is:

Pytor Ilych Tchaikovsky: Symphony No. 6 in B minor

I. Adagio - Allegro non troppo. (B minor)
II. Valzer: Allegro con grazia. (D major)
III. Allegro molto vivace. (G major)
IV. Adagio lamentoso. (B minor)

Notice the word finale not being written before Adagio lamentoso!  Tchaikovsky had put the slow movement as the last movement of his work!  Its structure is: opening - dance - virtual finale - slow!!!  It's worth pointing out the third movement is very energetic and exciting, and even concludes triumphantly.  If there was no rule violation, it would look like this:
(opening - slow - dance - finale structure:)

I. Adagio - Allegro non troppo.
II. Adagio lamentoso.
III. Valzer: Allegro con grazia.
IV. Finale: Allegro molto vivace.

(opening - dance - slow - finale structure:)

I. Adagio - Allegro non troppo.
II. Valzer: Allegro con grazia.
III. Adagio lamentoso.
IV. Finale: Allegro molto vivace.

Going back to the Mahler part, the fourth movement of his Symphony No. 1 should have been D major, because the original key is D major.  However, he chooses to use F minor instead - which was a dramatic break from conventional usage.  However, it ends in D major via a triumphant conclusion, and the symphony ends in D major, which we would typically expect to happen.

