The root of the issue is this -- our Mastery affects our spells in very different ways. It affects PW:S the most, obviously, and that is why many disc priests historically have chosen to stack mastery -- bigger shields. It then affects PoH decently since we have the guaranteed Divine Aegis proc (and, when it crits, it double dips). Finally it affects our other spells, but to a much smaller degree since it only comes into play on a crit.
This leads to some interesting stat weights. Mastery is better than even Int when it comes to shield strength (though Int is better overall because of mana concerns). However, for every other spell, even PoH, Mastery is our worst stat -- often by significant margins. This is a product of the requirement to crit as well as how well PoH and other spells scale with Haste.
The result is our best, most defining spell (PW:S) scales basically with only one stat, Mastery. Haste helps it somewhat (more shields per second), but not remotely as much as Mastery. Crit only helps the Glyph, so basically crit is worthless for PW:S. Every other spell we have, however, scales much better through every other stat.
I think the core issue is our Mastery doesn't really work well. In particular, Divine Aegis requiring a crit is just not a great mechanic. It makes gearing for Disc tricky and just leads to player confusion.
I would not presume to suggest what our mastery should be, but I do look at Holy Paladins with a jealous eye at times -- if almost every heal were affected by Divine Aegis, then our Mastery would be more appropriate. Regardless, it would be very nice to have our stats be more consistent... when stat weights from one secondary stat to another vary by as much as 4x for some spells, it makes gearing difficult. Lowering the impact of Mastery on PW:S and making it apply more evenly to our other spells would be outstanding. It also would be even more outstanding if crit somehow affected PW:S; I know that's tricky but that, too, would go a long way to improving our stat balance.
Overall Disc is actually in a really good place right now; it is fun to play with a lot of interesting tools. It's just the gearing and mastery side of things that make it a bit tricky and counterintuitive.