Over-production isn't a specific thing, it's simply when the production becomes the focus rather than the music - and what that takes entirely depends on the source.
Probably the best parallel is the film industry; every summer there's a bunch of terrible action films released that are literally nothing but a chance to show off the latest CGI. The obvious example this summer was Transformers 2 - the plot was completely secondary to the action sequences. They could have just released 2 hours of giant robot fighting (well, they virtually did) and people still would have gone to see it. The payoff for cheap thrills is that they're really short-lived experiences; the moment you leave the cinema you forget everything but the hottest chick and the biggest explosion.
Over-production in music is the same thing: you hear a song on the radio and it sounds big and punchy and gets you nodding your head, but when you listen to it through headphones in the dark you suddenly realise there's nothing to it; no depth, no soul, etc. Most people assume it's a bad thing, but that's just not true. The pop industry thrives on over-production, because people just want a quick fix to shake their asses to. You don't go to a club to sit down and think about the hidden depths of the lyrics to Kylie's latest hit. Modern R'n'B is nothing BUT production techniques - Snoop Dogg's "Drop It Like It's Hot" is a dozen samples of a guy clucking and a terrible sounding 80's synth lick with a guy talking over it. It's a "song" that has no melody and little more than 3 seconds of actual music - it's virtually beat poetry - but it's still a great track. The best pop songs are catchy AND produced to within an inch of their life - Britney Spears "Toxic" is a great example; it's bouncy and gets stuck in your head, but it's basically impossible to recreate live because of all the vocal effects and random layers of instrumentation.
I honestly don't think it's something that is that common in metal - metal largely relies on riffs and hooks to get your attention, and you can't really produce your way out of not having a good riff. There are still bands that are guilty of it though - early Linkin Park springs to mind, with all the vocal glitches (in fact the whole Nu-Metal scene was pretty guilty of hiding crap songs with clever production). Killswitch Engage rely on gimmicks quite a lot too (the telephone-stylee drum/guitar thing does my nut), but they tend to be structural rather than production-based.
The important thing is that metal bands rely on getting a dedicated fan base, and sugary instant-hit songs don't get you that. The metal albums that people tend to love are either ones that are full of great songs (regardless of the sound) or ones that take a while to sink in, either because of their technicality or their complexity; Tool, Mastodon, Metallica's earlier albums, "Reign In Blood" etc. Smashing Pumpkins were right on the edge with "Melon Collie..." - it was ridiculously grandiose and studio-based, BUT at it's core were a bunch of great songs that still worked on a single acoustic guitar.
Steve