Have been on 4
DCL cruises since 2002, but doing 2 RCCL cruises in '07, actually w/i a month of each other, one w/ DS and one w/o but w/ another couple (we cruised w/ them our first ever cruise, on RCCL, in '01). Cost was a factor, indeed--but so was itinerary available for a 4 niter.
I think w/ the other cruise lines you have to research the ships--some are older, some have much smaller rooms (generally, DCL rooms are larger than the peers on other ships, but then the $ is higher). There is a difference, and I expect that, but that doens't make RCCL a bad vacation--just not a Disney vacation. Is every land based vacation you take Disney? Sometimes you go Disney, sometimes you don't (for those addicts who only go Disney, okay, I know you're out there, I'm not flaming or DIS-sing you, but IMHO many people have some measure of variety in vacations).
Exporer, I believe, is a Voyager class ship? Wonderful for families, it will be a wonderful trip. We're doing the Radiance, and then w/o DS on the Enchantment, which simply was about as good a ship for a 4 niter as I could find that wasn't called Wonder and was spending time in the caribbean and not the Bahamas. The Wonder advantage is I believe it is the finest ship sailing 3 and 4 night cruises.
If there is something you notice that you miss about DCL, then you have to assign that feature a value...and determine if next time, you are willing to pay the difference for DCL...and willing to accept the itineraries available. I'm not gone for good--I have a late '06 booked on DCL, though I suspect I will want to move that to '07 when the dates open up--I want to go w/ the characters one more time before DS gets too old.
DCL reads the boards, they know about guests like us, and they have done their calculations and decided to do what they are doing. w/o expanding just yet. But since the Magic and Wonder aren't too empty, I think they're feeling good about things.
I guess though, as long as they continue to fill the ships, they don't care. 
