Almost embarrased to post this question here
Multiple processor cores are, at least in Aperture — Apple's equivalent to Lightroom — really helpful for speeding the work of handling lots of photographs. I assume Lightroom is also multiprocessor aware at this point. So from that point of view, the quad-core processors found in the 15 inch Macbook Pro and in most iMac models have a significant advantage over the dual-core processors found in lower-end computers.
The Retina displays also really do make a big difference in the laptops. The best part of it is the ability to scale resolutions. Want text that is easy to read? Pick the 1280x800 or 1440x900 modes. Running photo software? Pick the 1680x1050 mode. Looking over a spreadsheet with a lot of cells? Consider using the 1900x1200 mode. Except that they are all scaled modes, so they are actually much sharper than these modes would be on displays of native resolution. The 1440 mode in particular is sensationally sharp.
The full-size monitors, even though not Retina, are also much improved. The 94 and 100dpi displays of a few years ago (e.g. 24 inch 1900x1200 at 94dpi) have largely been replaced by 110dpi panels such as the ones in both the 21 and 27 inch iMacs — and they are very noticeably better than what they replaced as far as text is concerned, in my view, even though the text renders slightly smaller. The screen lamination process in the latest iMacs is another improvement, as it gives you the advantages of both a glossy and a matte display in one unit — relatively little glare, no fishbowl effect (with the iMacs in production until last year there was a gap between the glass and the panel, and I at least really found it like staring through a window), but also easy to clean and very resistant to the contrast problems that plague matte displays in brightly lit situations.
No full-size monitor as yet has the resolution scaling of the Retina displays on the laptops. I think that's a pretty big factor in favor of the Retina Macbook Pro. I suspect in the next year, we'll see 4K displays (about 3800x2200 resolution and up) that have scalable choices similar to the Retina, and with much sharper text as a result. The good news is that the Intel chipsets in current Macbook Pros, Mac Minis and iMacs support 4K, so likely (though not certainly) with OS X Mavericks these computers will fully support 4K displays on a Retina-type basis.