A Number of Lessons

9 5 2
10
A K 5 4
Q J 9 7 6
K 8 7 6 4 Q J 3
8 4 3 A J 9 7 6
9 8 10 3 2
8 4 2 5 3
A 10
K Q 5 2
Q J 7 6
A K 10

WestNorthEastSouth
Pass Pass Pass 2 NT
Pass 3 Pass 3
Pass 4 Pass 4
Pass 5 Pass 5
Pass 6 NT All pass

WestNorthEastSouth
Pass Pass Pass 1
Pass 2 Pass 2
Pass 2 NT Pass 4 NT
Pass 5 Pass 6
All pass

This hand is so plump with lessons that it could have been placed in any of about 5 different categories. The most obvious would clearly be the Balanced over the Unbalanced when it comes to choosing trump. The balanced suit here will allow slam, while the unbalanced will not. The Figure 33 would be germane for the pair that wound up in 6 no. Maybe a reference to Don't Sneer at the Minors would be appropriate, and lastly No Trump Slams (in a negative sense, as with the Figure 33). For as much as I like no trump slams myself, I have often pointed out that when you have a balanced fit (which I define as at least 4 cards in each hand), the odds are good, not certain, that you'll get an extra trick in a trump contract over no trump. Here with diamonds as trump, they not only serve as a spade stopper (when all trump have been drawn and clubs run), but you get an extra trick by way of a ruff for 5 diamond winners, which you cannot get in no trump and cannot get in a club contract.
But when I saw that one pair had smartly marched to their eminently makable 6 diamond contract while another not so smartly marched to an unmakable 6 no contract without even mentioning diamonds, I felt that the proper place for this hand lies under "Good Fits". A modest one, to be sure, and of course in a minor suit. But it works.
Finding and exploiting your fits is, of course, as important as finding the right level. Every bid has those two items, a level and a denomination, and it would be a fool's errand to try to determine which is more important. Here one pair got the right level but the wrong denomination for a miserable board. One cannot place a point-count value on a good fit and any attempt to do so would be fanciful. Here one might argue that the diamond fit is worth four points, which, in the form of the A of hearts in either hand, would bring the point count up to 33 and of course make the no trump slam feasible and preferable to the diamond. But on another hand it might be worth more and might be worth less. So I wouldn't factor in any point count but exploit good fits.
To be sure, this brings up exceptions. You want to exploit good fits in the major suits and balanced good fits in slams. Experienced players commonly shoot for the 9-trick no trump game, where feasible, in preference to an 11-trick minor suit game even with a good minor suit fit. And by emphasizing going for trump slams with balanced fits, I necessarily draw attention to how unbalanced fits will commonly serve you better in no trump, when you have all suits stopped. Here the hand plays substantially the same in clubs as in no trump. Well, it plays the same without a spade opening lead, and though there is a one-trick difference on a spade opening lead and that favors the club contract, the score favors no trump with 10 tricks.
So you have to bear in mind that a "fit" isn't necessarily the last word in your decision and that you won't necessarily follow a fit out to naming it trump. Nevertheless, fits are so important on the great majority of hands that I can only offer a couplet paraphrased from "South Pacific":
There is nothing like a fit; Nothing in the world.

It is easy to see that a spade opening lead would queer a no trump or club contract. Declarer cannot set up any heart tricks without losing the lead, and that allows the defense two tricks. But contracts in those denominations should be beat even without a spade lead. It might seem at a glance that a lead to the K Q of hearts would allow two heart winners to go with 4 diamonds, 5 clubs and a spade for 12. Not so. East must simply cover the 10 with the J. This may allow declarer to avoid a heart loser, but you'll get that back with 100% interest, for you have now cut declarer down to one heart winner. And that's not going to add up to 12.
That is to say, this would work in no trump and in clubs after three rounds of clubs were drawn. If done before trump were drawn . . . . hm-m-mm, come to think of it, declarer can make six clubs by making that play before trump have been drawn. Now, that's only on a defensive failure to find a spade opening lead. Declarer leads to the K Q 5 2 of hearts. If East goes up, declarer has 12 winners, sluffing two spades from dummy. If East ducks, now declarer plays the A and low in spades and upon regaining the lead, ruffs a spade in the short trump hand. Anyway, I hope the reader will see that in no trump that would work at any time in the hand. Just cover the 10 of hearts, and declarer can only thrash around in vain for a 12th trick.
In closing, let me touch bases with every category mentioned above. The balanced suit vs. an unbalanced (if the choice is between the two majors or the two minors) will almost certainly serve you better as trump if there is a difference. Here in a diamond contract, declarer can survive a spade opening lead by drawing trump in three rounds, then running clubs, sluffing a spade and heart from the closed hand. Declarer is now free to develop a heart winner. Of dummy's three spades, one will go on the ace, one on a heart winner and one will be ruffed.
The Figure 33? That's the hcp number (on balanced hands) you'll want to see to bid a no trump slam. You don't wanna miss the easy ones. Here South delimits his hand to 20 or 21 hcp's, a fairly common holding for 2 no openings today, and North should have no difficulty determining that they're just not there. Hm-m-mm. It would appear that South cheated by a point. But that's not an out for North who can see only about 30 hcp's combined, a significant shortfall from 33. No trump slams? I love 'em also. But on balanced fits (at least 4 trump in each hand) you'll quite often get an extra trick in a trump contract over no trump. Don't Sneer at the Minors? Self-evident in one bidding scheme. With two biddable minor suits, neither partner mentions diamonds.