My guess is you likely have two main problems (outside of properly calibrated your monitor):
1) some cameras default the photo to Adobe RGB - make sure they're saved in sRGB, which is actually pretty close to the Costco profile (not doing this would certainly result in images that are dull and less saturated)
2) the new iMac monitor is quite bright out of the box, so if you're adjusting your photos by eye, expect them to be too dark when viewed on a calibrated monitor (and to print too dark) - drop your brightness by half and try adjusting them again
I went through a few 24" iMacs before deciding that the screen quality was fine for regular apps, but not quite up to my expectations for photography (colour/brightness were uneven across the screen, appears to be due to the components used for this model, many threads about this). But not everyone is as picky as I am...YMMV
1) some cameras default the photo to Adobe RGB - make sure they're saved in sRGB, which is actually pretty close to the Costco profile (not doing this would certainly result in images that are dull and less saturated)
2) the new iMac monitor is quite bright out of the box, so if you're adjusting your photos by eye, expect them to be too dark when viewed on a calibrated monitor (and to print too dark) - drop your brightness by half and try adjusting them again
I went through a few 24" iMacs before deciding that the screen quality was fine for regular apps, but not quite up to my expectations for photography (colour/brightness were uneven across the screen, appears to be due to the components used for this model, many threads about this). But not everyone is as picky as I am...YMMV