1. You can't really compare the iMac to the rMBP. If specs were more important to you, it's almost always more economically efficient to buy a desktop over a laptop.
Except when the 21.5" iMac has specs that are almost identical or inferior to the 15" rMBP. 27" iMac is the only iMac this generation that has any real specs advantage, but only the top-end config does. It seems crazy, but the 15" rMBP, even the base model, has specs that are almost comparable to a base 27" iMac. Only lacking a little bit in CPU performance.
2. rMBPs tend (or so I've heard) to have a lot of problems that can and do go wrong with them.
This I can agree with. Had a slew of problems with my rMBP.
3. rMBPs are not user upgradeable (you have to buy Apple's extremely expensive hardware upgrades then and there).
4. rMBPs are a PITA to fix. Everything is soldered in, so if anything breaks, you have to ship it in to Apple (or at least go to a certified AppleCare store).
It's the same story with the 2012 iMac, actually. There's not much you can upgrade easily on the iMac. In fact, RAM is probably the only thing that you can update on the iMac without having to pry the screen off.
5. Other than the screen, there aren't any real benefits to the rMBP. Officially, they can hold up to 16GB RAM. But unofficially, so can the other MBPs.
The rMBP actually holds more benefits elsewhere with weight, overall device volume, cooling system, speaker system (more than 2 speakers), more Thunderbolt ports, and port placement (USB slots on either side and not just one side) compared to other MBP.
6. The rMBP has no optical drive. I tend to use CDs frequently.
The redesigned 2012 iMac doesn't have any optical drive, either. In fact, I think the classic unibody MBP is the only Mac in 2012 that has optical drives. Looks like those are going fast.
7. Obviously the price of the rMBP is much more expensive than other Apple laptops.
This is actually not true. Configure a 15" MBP to the same specifications as an rMBP, and it turns out the rMBP is cheaper.