I, too, have been researching other lines to try and figure out if another line would make us happy. Get one of the big cruise guides like Frommers. You get an overview of each line. I get the overall impression that there are many lines that can compare to
DCL.
Royal Carib, Costa (bad food reviews), Princess, Carnival, Holland, Celebrity and Norwegian all have ships that compare well to DCL, food, public, dining, etc. HAL and Celeb have cabins in the 175-185 sq ft range (DCL is 225 approx). RCLs are pretty darn small (120-160). Princess 160-250, but pricey for the larger.
Royal Caribbean has the most ships and has Caribbean itineraries in the summer months. Celebrity sounds great, but moves their ships to Alaska and Europe during the summer months, as does HAL. But these three seem to get quality ratings more on par, or better, than DCL.
Within each of these companies the ships and ammenities vary greatly. I am mostly running into the fact that when the prices are bargain via the web, the cabins are too small to hold four persons. None of these lines have the "nearly every cabin hold 3-5" persons as does DCL. The cabins that do are few, even on the monster ships.
The best approach, when hunting, is choose the date and place, then see which ships are going where YOU want to go. Then look for a cabin size that fits.
Haven't researched Princess much yet. Nor Norwegian, because I want to be waited on by the same servers, not into standing in line at the free choice restaurants. (Did NCL in the past before "free style".)
This month seems to be the time for lots of cruise shows. We are going to go to a show that has a lot of companies on display, from the dirt cheap (Carnival) to the ultra (Raddison, Crystal). Go to one of these and you can pick up plenty of the guides.
Check out the web sites, try plugging in your party size of four or more and you nearly always get bumped to call them anyway! Check out the cruise critic reviews and links for lots of opinions.
Carla