This is a great question. I would think for a first cruise, any line would be fine since there's nothing to compare it to. Our first cruise was on Commodore lines Enchanted Isle. We loved this cruise and have no regrets. It was terrific. However had we shipped on say Princess or HAL before we wouldn't have liked Commodore nearly as much since the facilities just weren't on a par at the time. These were the days that Princess was still sailing the old Love Boat sized ships which were about the same size as Commodore.
I guess my point is is that many experienced cruisers might say "Don't ship on NCL, the other lines are much better", a first time cruiser would love NCL. Then of course after they fall in love with cruising (and of course they would) they can try other lines later and be able to compare.
I used NCL as an example. We've also sailed on RCCL, HAL, Celebrity... and I would sail NCL again without any concerns, as well as the other lines still around.