One major issue is in graphics work where the HD 4000 in the 2012 Mini is only getting older and the Iris chips that Apple is using in the higher end 2014 Mini (and likely higher end configurations of a 2016 or 2017 Mini) are a significant upgrade. The HD 4000 is quite capable of running a desktop environment at high resolution and includes multi-monitor support but it struggles with 3D work, especially on software that is newer than 2012.
I rely on mine for gaming and it is a fine chip for Indie titles and older games but falls apart with virtually anything new. The last major release I have played on it with a reasonable frame rate and visuals was Borderlands 2, though I am sure other gamers on this system might have experience with newer titles. I'd been thinking of trying Fallout 4 on it but I loathe the idea of playing that game on low with a resolution below 1080p just to get playable FPS. It goes without saying that if you aren't planning to run these kinds of programs, the HD 4000 should remain adequate for the next couple of years. This chip is capped at 2k resolution (2560x1600 IIRC) however, so if you are planning to upgrade to 4k+ in the next few years you will want something newer.
My personal plan is to keep my Ivy Bridge mini until 2017 when cannonlake becomes widely available. Unless Apple does something really surprising with the minis (like offering user upgrades and quad core CPUs again), I'll likely retire it for an iMac. I don't figure on it being a bad computer in 2017, but I will be looking to upgrade from that HD 4000 to at least a 4k display. I'll probably turn the computer into an HTPC or give it to my parents after that.
Great post. Mine wasn't too bad for some work in Unity and Unreal Engine, FWIW, it did make a noticeable difference when I went from a '12 i5 to a '12 i7 (same RAM/SSD) in those apps. That being said, if I were to have pursued that direction, I would definitely have moved to a newer machine (probably a rMBP '14/750M, or a '15/390X).
For gaming, we just use a console. There's always something good (even if some "PC only" titles don't make it), and the dedicated gaming hardware is pretty stout (and we can chill in front of a big TV on a sofa
I'm with you on keeping this machine though, I think it'll easily have solid life - outside of hardcore gaming - well into '17. I mean, right now, I've got 2 VMs running with dozens of apps, services, all the OS X native apps and services and it runs without a hiccup!