How do you choose a cruise line? Where is the market going?

In another thread, we went way off topic and ended up discussing something that may be better as a stand-alone thread. Specifically, where is the cruise market going, and what are the real differences between say... Carnival, Princess, and Holland American?

I was under the impression that Carnival used to be a youthful party ship. I had not sailed for a while, then last year was on a 5 day Carnival. While I noticed they had certainly opened up cruising to a lot of people who might not have cruised before due to their pricing, when comparing to my previous Princess and HAL cruises, I honestly didn't feel a huge difference in the product. That is when I compare the quality of the staterooms, food, basic services to HAL or Princess.

Yes, I certainly think HAL and Princess had better food. But if it made the difference in going on 25% more cruises in my life, or instead of one cruise every 12 months vs. every every 9 months, was the difference that important?

My guess is that I need to try HAL and Princess again soon to be a good judge. I just got a flyer from Princess offering cruises at $90 per day. My next Carnival Cruise is at $80 per day. Seems like the market is tightening.

Where is the market going? Are the previously "low-end" lines getting that much better, and what are the previously "high-end" lines doing to be able to reduce their costs and pricing?

We all are different of course in what is important to us. So what is important to you in a cruise, and how do you decide which line to choose when looking at your next cruise?