I hook my rMBP up to my 24 inch 1080p sometimes (cheap monitor and less ppi than an iMac screen obviously) and from where I sit it looks just as good. However some pics when I look at them in both screens...I do see some better clarity at times. As someone who does photo editing, retina is AMAZING but not needed. As someone who does video editing, not a big deal IMO, and frankly I could use the 16:9 for my timeline. For gaming...man is it beautiful at 1800p...but I usually play at 900p/1050p and sometimes 1200p cuz 1800p heats up my rMBP pretty damn hot. I don't wanna get anything non retina anymore, but it's not needed, and won't be the standard for a while. I play LoL and none of the menus/pics are retina, I can't tell until I lean in but bleh, looks pixelated when I do
Also, when I compare my 1800p, my 1080p, and my 720p screens, the 1080p is always in last cuz it's a matte finish Asus I got for like $100. There's something to be said for screen quality as well, and iMacs have that regardless of retina.
Also to my point earlier, I have a first generation rMBP and I LOVE it, but I do have that weird vid card flicker bug. Haven't sent it in yet, and frankly don't know if they'd fix it anyways since they seem to dodge video card repairs, but regardless of bugs I've enjoyed this beautiful machine everyday. Having said that I'm also looking at iMacs, perhaps even a 1080p one