I've only been on one cruise of the "standard" sort. It was a Holland America deal on what to me seemed an enormous ship (1,400 pax). My only comment is "Never again!!!" just because of this. And frankly, there weren't all that many slowpokes on the ship; it just seemed that way.
Otherwise, I've been on a couple of Lindblad cruises. Despite their extremely high cost, they were worth it. When you have only enough passengers that you can practically know everyone's name by the end of a week, the ship is loaded with kayaks and Zodiacs for your enjoyment, and there are no "formal dinners" to think about, it's a completely different experience.
On my Alaskan expedition with Lindblad, the ship stocked every single style of beer made by the Alaskan Brewery, which is one of my favorites. How can you go wrong with that? And the National Geographic photographers on board were absolutely awesome. Incredibly helpful, even if you didn't know a Leica from a lens cap.
We have already booked a Lindblad cruise to the Galapagos for next year, and I'm looking forward to it. This is from the guy who (after that Holland America experience) said "I'll never set foot on another cruise ship."