Assorted comments, not aimed at anyone in particular, written from the perspective of someone who less than 2 weeks ago switched from a 2008 iMac 24" running Snow Leopard to a 2012 iMac 27" running Mountain Lion:
• The idea of an AIO computer is outdated. At the least, everyone that uses an iMac for any serious purpose should have one or more external HDs for backups. Ideally, at least one of them should be capable of being used as a startup drive.
• Apple has never made every port as accessible as it should be. Anyone who occasionally uses headphones with an iMac knows what I mean.
• Consumer grade slot-loading DVD drive mechanisms are not very rugged or reliable. It isn't just the ones Apple has used; it's all of them. They all use cheap, stamped metal parts & actuators just barely powerful enough to do the job. I can't remember the last time I saw one that included a manual eject override (usually accessible through a tiny hole) in case a disc got stuck & could not be ejected normally.
• I wasn't happy that my new iMac didn't have a built-in DVD drive but I got over it. The slot-loader in my 2008 iMac had become cranky: sometimes it would not recognize a disc the first time it was inserted; sometimes it struggled to eject a disc. I was afraid that it wouldn't be long before a disc got stuck in it, necessitating a costly trip to the shop or a risky DIY teardown to fix or replace it.
• Mostly because of that, I'm starting to realize that an internal optical drive isn't such a good idea. If an external one fails, I can just replace it. And I am not limited to what Apple offers. I can get a decent Blu-Ray burner from OWC for about $100. For offsite & archival backups, the much larger capacity of BR discs is a better fit for me.
• The thinness of the new iMac is obvious. What was not until I used it for a while is how amazingly quiet it is & how cool it runs, even under heavy load. It also feels more sturdy than any iMac I have ever used & somehow has richer, deeper sound even though there is less room inside for speakers. I don't know how Apple managed all that but I suspect the thinness is more a consequence of an improved, "less is more" internal design than a design goal in & of itself.
• Every design is a trade off among mutually conflicting goals. I would love it if the new iMac had a built-in optical drive, all the ports my old one did, & so on. But if that means compromising on the striking improvements this one includes, I would not be a happy camper. It is easily the best iMac I've ever used.