This question cannot be answered briefly. The "Sacred Steel" doc video I directed for Arhoolie probably does it fairly concisely (I hope), and presents many musical and contextual examples.
As far as steel guitar technique a few things come to mind:
The instrument is often played linearly along the fretboard, i.e., extended passages are played on one or two strings. This helps give a singing sound and enables the steel guitarist to execute melismas (see below). If you want to sound like Aubrey Ghent, for example, you pretty much have to play that way.
The extensive use of melismas. For example, the word "Jesus" has two syllables, but a sacred steeler (imitating the black gospel vocal style) might make six or eight notes of it.
Moans, imitating the low-register vocal moans (which are usually melismas), played on the bass strings. Sacred steelers do this extensively, especially when playing in church. To my knowledge this is not done in other musical genres--at least to anywhere near the extent the sacred steelers do it.
Rhythmically strumming (which they call "framming") the pedal steel guitar. Pretty much unheard of in other pedal-steel guitar genres. Many sacred steelers play a lot of percussive, syncopated strums.
The common use of a wah pedal as a tone (timbre) control, as opposed to making the wah, wah, wah sound. Often combined with a degree of distortion. While rock and blues steel players may use a wah pedal sometimes to get a similar tone, the Jewell Dominion style, as established by Lorenzo Harrison in his later years, is built on this being the most common tone.
Playing pedal steel in variant of E7 or E major tuning as opposed to the E9 chromatic and C6 tunings and copedents commonly used by country and western steelers.
The style(s) of accompaniment--rhythm guitar, bass, and drums--contribute significantly to the total sound. Jewell Dominion rhythm guitarists, for example, have a large number of shuffles, riffs, and boogies that are essential to producing the total sound. Take away the distinctive backup and it sounds more like blues or rock and less like "sacred steel."
There are many phrases or licks that are identified with certain historically important players such as Henry Nelson or Calvin Cooke, for example, that are distinctive, if not totally unique.
Grooved bars:
When pedal-steels began to become popular with House of God steelers in the early 1970s, some of them (Ted Beard may have been the first) had grooves machined into large diameter, long pedal-steel bars. Sacred steelers, including those who play pedal-steel, lift and tip the bar frequently and use a lot of left hand blocking. The grooves make that a lot easier when the bar is large and heavy. (Most country pedal-steelers rarely lift the bar). There are many sacred steelers who play 10 and 12-string necks using a Stevens bar, or something similar.
Like I said, it's a long story. I'm running out of time today...
Hope this helps some.<font size="1" color="#8e236b"><p align="center">[This message was edited by Bob Stone on 16 October 2006 at 02:44 PM.]</p></FONT><font size="1" color="#8e236b"><p align="center">[This message was edited by Bob Stone on 16 October 2006 at 02:51 PM.]</p></FONT><font size="1" color="#8e236b"><p align="center">[This message was edited by Bob Stone on 16 October 2006 at 03:43 PM.]</p></FONT>
