I've talked about /Stone Armor on the SCORE server in the past as well because, I too, really love the concept of /Stone Armor, however my idea.. didn't really go well with people. I have since refined my idea a little bit since then and, perhaps, people here might think a little bit differently about it.
To give some background, back on Live, I had a level 50 Stone Melee/Stone Armor (Along with the same character as a 50 Tanker. This was before we could just.. switch sides. I gave up on the Tanker after that.) I've also played several other brutes, including a level 50 SS/WP brute that I love to bits and whom is extremely tough in her own right with a pretty kited out build. In terms of my re-leveling experience between the two characters on Homecoming, the only thing I felt lacking on the SS/WP was just damage until I got Footstomp as SS has no AOE until that time. (Unless you grab cross-punch and/or spring attack)
Re-leveling my SM/Stone brute (Whom is now 48 with nearly my full planned IO build minus purple sets) was just.. yeah, everything that I remembered it to be back on Live. Survivability was.. okay, compared to my SS/WP, but that, in-part, came from Fault being the awesome power that it is. Thinking back and actually looking at the numbers in Pines, I'd say that /WP gets pretty comparable (mathwise) /DEF and /RES. One gets an always-on (but smaller) HP boost, while /Stone gets Earth's Embrace. What isn't comparable is the amount of regen /WP gets in comparison. With just one mob in range, you quite a bit more HP/s then what /Stone gets while running Earth's Embrace. (Which is not perma with an SO build) EE also heals you, yes, but the amount of healed HP you get is only worth the difference of about 30 seconds of /WP's extra regen with one 1 mob in range. /WP nearly doubles it's regen with 10 mobs in range. It also debuffs those mobs with a small, but still there -3.75% Tohit.
So, technically, until you get Granite, you're actually behind /WP in terms of survivability. /Stone has nothing that helps you in a crowd of mobs like /WP does. Rooted does give you some debuff resistances that /WP doesn't get which is nice but doesn't really warrant the debuffs that you get in exchange, especially in modern play. While /Stone has decent END drain resistance, there's just nothing that helps it's large END requirements to run all of those toggles. (Yet /Elec gets both END/Rec resistances along with END powers.) Mudpots is especially nasty at 0.78 END/s (Verse 0.52 END/s for other damage auras, but it does have the slow/immob in it, although sometimes it feels like that part doesn't even work and mobs just run through it.)
There's also the other issues with the set, such as lots of toggle juggling that other people have mentioned. Particularly if you actually want to pop in and out of Granite. (And because of this, you actually are running an END discount while in Granite because you're running less toggles... Which seems like a silly mechanic to me.) You are, in fact, actively discouraged to not get out of Granite.
With all of that being said, here are my actually ideas to improve /Stone without effecting the flavor or how the set works too much from what we have now. Keep in mind, these are brute numbers I'll be suggesting since I'm more familiar with them.
1) Strip 5.63% RES off Granite and give it Stone Skin. As long as Stone Skin is slotted for the same RES enhancement as you would Granite you still get the same over-all amount of RES when in Granite. You'll just have more RES when not in Granite. Definitely helps the lower-end survivability while not changing high-end at all. This also makes Stone Skin more comparable to High Pain Tolerance and True Grit.
2) Rooted should definitely come sooner then Mudpots. If we must keep the rooted debuff on it, then it should either be buffed in some way or part of the -runspeed debuff should be resistible. (Or just flat reduce it) That gives another way we can build around the debuff, at least. (Basically using Winter sets to resist some of the -slow debuff) Although, in my humble opinion, I think if it had only the rooted part of the debuff but little or no -runspeed, then it would be about right. It would be similar to the effect of Grounded. -Runspeed, I feel, should just be mostly from Granite.
3) In order to reduce toggle juggling and normalize END usage and basically give the set better QOL in it's handling I suggest this: Make Granite, Crystal, and Brimstone Armor the only exclusive toggles of the set. How would this work?
A) Shave ALL of the S/L defense off Granite. (Yes, all 15%) and let Rock Armor stack on top. Yes, you'll have less S/L defense because of this. Yes, you'll use more END because of this. But, what could you get in exchange, you ask? Being able to run Minerals, if you want, on top of Granite. I feel that's a pretty good trade-off. Granite more-or-less stays the super-tank toggle and you now can get Psi DEF while in it. You still wouldn't have Psi RES though. Perhaps some toxic RES could be removed from it to further balance this, if needed?
B) Since Crystal and Brimstone would be exclusive to each other. Each can have a different theme to them. Crystal could then be the tank-lite toggle. Buff up it's numbers (Adding in DEF it doesn't have) and maybe even give it a PBAOE debuff aura like some of the other sets have. Or give it extra END recovery or both?
C) Brimstone could then be your Offensive-leaning armor toggle. Buff its RES up minorly (Adding what it doesn't have), but have it boost the damage of Mudpots, or even give the Fiery Embrace mechanic, definitely seems in-theme. Give it some slow resistance, even. (Which could make you run faster if the -runspeed was kept on Rooted, but part of it was resistible. Snergy~)
Effectively, this idea makes those 3 toggles into stance toggles: Heavy Defensive (Granite with the heavy debuffs it already has), Light Defensive (Crystal with no debuffs and some buffs), Offensive. (Brimstone with less overall protection, but with more offensive abilities tacked onto it.) Rock Armor and Minerals could always be used under those three, so you're only having to switch between those 3 toggles, with no toggles dropping. Far simpler. It's even better since you -wouldn't- have to take all three, if you didn't want too. You'd have a choice in which ones you actually want to keep and use of those three armor toggles because they couldn't be used at the same time. The cooldown on all three of the toggles could be increased for a further balance point, as well.
The only people I'd assume whom wouldn't like this change (And have a decent reason for it), are the ones whom do already layer Crystal and Brimstone together and like that look. For them, couldn't alternate FX be added to give that layered look on either Crystal or Brimstone by themselves?