Carnival isn't a bad line. They are simply a very clear example of the maxim "you get what you pay for". Carnival is generally quite a bit cheaper than the other major cruise lines. This is because, for a similar-sized ship, they will cram in another few hundred passengers. Most often, these are families, since those with 3-4 kids can't really afford to go on any other cruise line. (The Disney line is often twice as much per person as Carnival.)
What you get is a TON of kids running around, fewer crew and attendants per guest, longer lines, etc. This leads to complaints, and lower ratings than other cruise lines. However, if you signed up, it was likely for the lower fares. Caveat Emptor!
(Personally, I prefer Holland America. Generally an older crowd, so quieter, less competition for the jet skis and other cooler port excursions, shorter lines with the lower passenger-to-crew ratios, and often slightly better food and more mature or conservative entertainment... and about 500-800 fewer kids per boat. That's good enough for me. YMMV.)
I’ve only done Royal Carribean cruises (Explorer, Navigator, and Freedom), and they were fantastic. The wife and I really want to do the Oasis, but now we have kids and the cost rises exponentially, unfortunately.