An example of such an unbalanced hand is: ♠9 ♥AK73 ♦K762 ♣QJ84. When partner responds 1♠, as he surely will most days, you need to be able to make a non-reverse and non-notrump rebid. Therefore you start with 1♦ and rebid 2♣.
But suppose the major suits are reversed: ♠AK73 ♥9 ♦K762 ♣QJ84. Now you can happily open 1♣ and you will have no problems with your rebid whatever partner responds.
What about stronger hands? For example, ♠K ♥AK73 ♦K762 ♣AJ84. If partner responds 1♠, won't you want to rebid 2NT to show your strength and overall semi-balanced shape? You might even do this with ♠9 ♥AK73 ♦K762 ♣AKJ4. To open this hand 1♦ and follow up with a jump-shift to 3♣ over partner's 1♠ seems to me to be vastly overstating the strength of your minor suits.
Hands in the middle range (15-17) are slightly trickier but generally have to follow the same rule as the weaker hands. Assuming you're not tempted to open 1NT with an honor singleton, as so many people are, you will open ♠K ♥AT73 ♦K762 ♣AJ84 with 1♦ and rebid 2♣ over partner's 1♠ response just as you would with the first hand. I suppose there might be hands with concentrations of honors where a reverse would be tempting, such as ♠9 ♥AKQ3 ♦8762 ♣AKJ4 where 1♣ followed by 2♥ wouldn't be such a terrible lie (yes, I know you are supposed to have another club).
So, assuming equal length in the minors here are my "rules":
- if balanced, open 1♣
- else if strong (18+), open 1♣
- else if you have biddable spades, open 1♣
- else, open 1♦
I'm well aware that this is going to be a controversial subject -- I expect comments. Please keep them relevant, though. I'm particularly looking for any gaping holes in my proposal. Is there something obvious that I'm missing?