[Thranduil]

It's a pretty clear 2♣ opener for me. 7 playing tricks, 3½ losers. With just ♥KQxx in partner's hand and no other points, 6♥ makes. I'm not risking 1♦ getting passed out here.

Considering 2♣ is not an absolute force, but the general forcing opening in 2/1, a normal minor suit rebid after 2♦ should not necessarily be an automatic game force - partner must be allowed to pass with an atrocious zero points hand.

If you have a slam in your own hand even opposite zero points, just bid it yourself or make a jump rebid of your suit after 2 clubs.

[hrothgar]

One comment that no one has made yet

Single suited hands with Diamonds are very awkward when you open 2!C because the rebid consumes a whole lot of space
Reverse type hands with Diamond and Hearts are incredibly awkward because your second rebid will force you to the 4 level.

People are often particularly conservative when then chose to open 2!C with this shape.

[mycroft]

• If your partner will not bid 1♥ with ♥KQxx and out (as opposed to ♥K ♠Q, but even then some might think about it), then maybe you have to open 2♣ with this hand.
  
• "How can partner pass with ♥7xxxx, when I have the ♥KQ?" (with the rest of the hand)
  
• 2♣-2♠-X (bad hand!)-4♠. Your call? Even 3♠...
  
• for"ever", 2♣ openers have been forcing to *suit agreement*, 2NT, or game (opposite zero). i.e. you can pass 2NT (if natural), a suit bid-and-rebid, or a suit that you bid naturally that partner raised. 3♦ is none of those things, and that's why people look for reasons to *not* open 2♣ with primary diamonds, never mind primary diamonds and a second suit(*) - because opposite zero, you can't get out below game with your "not actually game-forcing" hand.
  
• There are good reasons for 4.
  
• I seem to be a shill for Skid Simon, but he really did say it all back in 1945: "bad players overbid their good hands, which is *usually* okay, because bad players also underbid their bad hands. So it usually works out." But when the bad players get hands they can't underbid...

(*)Good to know, hrothgar, that I've been promoted to "no one". :-)