What should have been a simple task just ate an hour of my life (and, trust me, the marginal value of an hour of your life is an increasing function of age). I'm trying to compare two parts of a text file, and I wanted to use a split view in gedit. That's not a feature of gedit, so I sought out and found a plug-in, GeditSplitView, that should do the trick. I downloaded it and installed it to ~/.local/share/gedit/plugins (after creating ~/.local/share/gedit and the child plugins folder). That I had to create the folders was a bit surprising, as I was sure I'd previously created them for a different plug-in (now gone missing). I keep my home folder tree on a separate disk partition from everything else, so upgrades (such as my recent installation of Mint 14 Nadia over Mint 11 Katya) should not disturb anything in the home folder. Well, whatever.
After installing the plug-in, I restarted gedit - and discovered it was not seeing the plug-in. Hmm. As an experiment, I installed the gedit-plugins package (a set of optional plug-ins) from the Ubuntu repositories, using Synaptic. Gedit didn't see those, either, which sent me off on a fruitless expedition of web searching.
It turns out the problem is simple, if not intuitive. There was a change in plug-in formatting (and naming conventions) between gedit 2.x and gedit 3.x. The GeditSplitView plugin requires gedit 3.x. That brings me to the daffy (to me) part. Mint, as it installs itself, uses the Ubuntu 12.10 "Quantal Quetzal" repositories. (Mint is a fork of Ubuntu.) It lists version 3.6.0-0ubuntu1 as the current version of gedit-plugins (which is what I installed unsuccessfully) ... but it comes with gedit 2.30.4-ubuntu1 (and the corresponding version of the gedit-common package) preinstalled, and lists those as the current versions. So it's giving you incompatible versions of gedit and gedit-plugins as defaults (and lists no other options).
Once I sorted that out, a quick search turned up instructions on how to uninstall gedit 2.30 and install gedit 3.6.1 in its place. After that, I was able to install GeditSplitView easily, and gedit had no trouble finding it.