I'm not really sure what John is saying about the diminished chord giving the minor key its minor sound or the progression being backwards compared to the major key, but I'm happy to explain a bit more about why minor keys are so variable...
I don't want to confuse you or anyone else reading, so this is strictly for those who are interested, and for those who are already familiar with scales, intervals, and chords. It is as useful for musicians as art history is to artists...
Basically the reason is that there are lots of different minor scales...
As you know, the relationship between the major scale and the relative minor scale is simple: they are the same, and produce the same chords. But, the numbering of the chords is changed because the root is different. For example:
C major:
I - Cmaj7
ii - Dm7
iii - Em7
IV - Fmaj7
V - G7
vi - Am7
vii - Bm7b5
Note that the interval structure of the scale is 1-2-3-4-5-6-7. Also note that the V chord is a 7th chord, which resolves really well to the I chord.
A minor, aka the Natural Minor scale:
i - Am7
ii - Bm7b5
bIII - Cmaj7
iv - Dm7
v - Em7
bVI - Fmaj7
bVII - G7
Note that the interval structure of the scale is 1-2-b3-4-5-b6-b7. Note also that the v chord is a m7 chord, which doesn't resolve nearly as well to the i chord as a 7th chord does.
So, what people did is turn that Em7 into an E7 by raising the G to a G# (compare the two chords on the guitar, it's the easiest way to see it). This gives us a new scale: A-B-C-D-E-F-G#. Its formula is 1-2-b3-4-5-b6-7, and is called the Harmonic Minor scale. It produces the following chords:
A harmonic minor:
i - Am(maj7)
ii - Bdim7
bIII - Cmaj7#5
iv - Dm7
V - E7
bVI - Fmaj7
vii - G#dim7
As you can see, you start getting some pretty odd chords, so this scale is most often combined with the straighter sound of the natural minor scale to avoid the augmented and diminished chords... Also, it has the tone-and-a-half distance between the F and G# (i.e. the b6 and 7) which is an awkward melodic jump. So, people created the Melodic Minor scale with a 6 instead of a b6 to allow more flowing melodies, with the strong resolution allowed by having the 7 (G#) instead of the b7 (G) in the natural minor scale.... Originally it was different when played ascending and descending (descending it was just the natural minor), but the ascending version is used in jazz, and it's 1-2-b3-4-5-6-7...
A melodic minor:
i - Am(maj7)
ii - Bm7
bIII - Cmaj7#5
IV - D7
V - E7
vi - F#m7b5
vii - G#m7b5
Again, it has strange chords, so it is often combined with the natural and harmonic minor scales.
But, there are still more minor scales. The most important ones are some of the modes of the major scale: the Dorian and the Phrygian (the natural minor is the third minor mode, known as the Aeolian). Here they are harmonized:
A dorian, which has the formula 1-2-b3-4-5-6-b7
i - Am7
ii - Bm7
bIII - Cmaj7
IV - D7
v - Em7
vi - F#m7b5
bVII - Gmaj7
This is common in jazz, funk, and bluesy minor progressions.
A Phrygian, which has the formula 1-b2-b3-4-5-b6-b7
i - Am7
bII - Bmaj7
bIII - C7
iv - Dm7
v - Em7b5
bVI - Fmaj7
bvii - Gm7
This is common in darker music, as well as jazz.
All of these scales and modes can be thought of as producing minor tonalities (although modes don't strictly produce keys, but it's the same idea), and are often combined and interchanged within songs to produce specific moods.
That may be more info than you were after, but at least it shows why there aren't hard and fast rules regarding minor chord progressions...
James
P.S. That was a lot of typing, so if there are typos, let me know...
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