One generation would make a huge difference if it were 1988, but these days it pretty much is almost irrelevant.
Let's look at the features people are hoping for from a refresh:
- Ivy Bridge vs. Sandy Bridge: maybe a 10% performance difference for applications that max out the CPU. Saves a few watts of power too but probably only amounts to $1-$2 on a power bill *per year*.
- USB 3: definitely a huge nice-to-have but not really necessary. Name one thing you *need* USB 3 for that can't be done with Thunderbolt (albeit for a somewhat higher cost).
- SSD: you can get factory SSDs with the current iMacs. If it's too expensive then you can fairly easily install your own using existing instructions and kits, or you can plug one in externally via Firewire or Thunderbolt.
- Reduced screen glare: nice, but people have been getting along fine for years with the current amount of glare. Reposition your desk or lights if you simply can't deal with the glare.
- Lack of optical drive: put a piece of tape over a 2011 iMac's optical drive and pretend you have a 2012 iMac.