Hi jkoseattle,
I wondered exactly the same thing for quite a while, and here's how a colleague finally talked about "glue" compression that finally made sense to me: When people refer to "glue" they're talking about the impression of a compressor filling in the spaces in the dynamics of your music. Try this: take your master bus and place a compressor on it with a mild ratio (generally no higher than 4:1, but YMMV), slow-ish attack, medium release and threshold turned all the way up so you aren't compressing anything. Now, without looking at the gain reduction meter, start turning down the threshold and listen to the spaces in your song. By "spaces", I mean the spots where the dynamics are softest...where kick and snare aren't playing and the vocals aren't terribly forward, essentially the spaces between transients. As you turn down your threshold those spaces will start to kind of fill in as the compressor works. Hence, "glue": the impression of filling in the spaces between things while leaving the transients intact.
 
Good luck,
Dean

These guys pretty much all nailed it. To me, high threshold is the key. If you need to use two stages to "pre condition" the sound, so be it. The high threshold and subtle ratio brings soft levels up and tames the peaks, so you get a more even sound. If you are looking for a bus compressor that specifically refers to "glue" in it's product description, IK multimedia's Bus Compressor comes to mind, although I will say it sounds similar to the PC-4K.

I'd add the one point that no one's mentioned yet, but which you've probably already suspected: glue compression is entirely optional.
 
Back in tape days, it wasn't optional. It was built into the tape. Many people had a hard time switching over to an all-digital environment, and a big part of that was they were missing the tape effect. Tape was usually driven into saturation, which applies compression - "glue". And that makes the mixer's job easier. Tape sims and glue compression are an attempt to recreate that effect.
 
But it's not necessary. You can choose to embrace the clarity of digital audio and provide your own "glue" through level automation, track compression and EQ.
 
We can do things now that were only a dream 40 years ago, such as applying dynamics control to every track (yes, one console manufacturer did provide primitive compressors on every channel, but most of us couldn't afford those). Back then, you might only own one pair of nice compressors, so you had to carefully choose which track or bus to use them on. Now you can have any number of compressors. Back then, putting a fully-parametric equalizer on every track was not an option, now it is. Back then, volume automation was limited to imprecise broad strokes; now you can automate the tiniest details with millisecond precision. 
 
I do use bus compression, sometimes. But when I do, it's an admission that I've not done a good job on the mix and need a crutch. For those of you who routinely use bus compression, try this experiment: pull up an old project (old enough to hear with fresh ears), remove the comp and volume-match the original. Then A/B both versions of your mix and note how the uncompressed mix pops to life.