Judging the Dell vs iMac strickly on the merits of hardware
Dell Disadvantages
It appears to be limited to the S series I5 & I7, the lower power, lower clocked (and slightly cooler running) versions where the iMac has no trouble using the full desktop versions which feature higher clocks. Granted, the iMac is still limited to 2nd gen I series processors hence the clockspeed and benchmark disadvantages--but once (or I guess if) the iMac gets its 2012 refresh soon and moves to 3rd gen Ivy Bridge, you'll see the clockspeed advantage back with the iMac which should sway the benchmarks advantage back to the iMac.
It only has 2 DIMM slots so if you go with 8GB (which will come as 2x4GB DIMMS) and later want to upgrade to 16GB, you have to toss the original memory. The iMac has 4 slots, so you would be able to add to the original memory one time without having to toss the original DIMMs.
The $1999 top end model tops out with an NVidia 640M, a less powerful GPU than even the new MBP retina comes with. This doesn't make any sense at all since the 650M has been available for awhile now, unless it's a thermal issue like the processor.
I don't like that it comes with a wireless keyboard/mouse combo that uses a USB transceiver dongle instead of bluetooth if you select or build a model that has bluetooth built in. You're just wasting one of the 4 available USB ports for the keyboard/mouse dongle. Also you have to pay another $50 to get a trackpad, and it ALSO uses a USB dongle--there goes another USB port and you've only got 2 left before having to break it out with a USB hub. Not to mention the keyboard/mouse combo that come with it look plasticky and cheap.
It has no Thunderbolt capability whatsoever.
The Dell looks like it's plastic sandwiched between the glass front and aluminum back. Maybe this is why they are stuck using the lower end GPU and S versions of the CPUs, they're having trouble with the thermals in this design?
Dell Advantages
It has a Blu-Ray combo drive.
It has a built in HDTV tuner.
It looks nice (yet still no height adjustment, just like the iMac).