Going point by point:
Penril wrote:
- The mechanic has to be played. Not just "spam Ice Spikes to 100 then forget about the mechanic" like it currently is.
For this the best way is to give DOUBLE incentive, a drawback as well as a convenience, simply giving them an harsher drawback will turn into a "don't hit 100 or you'r screwed" if you give them something good by dropping their comb/Dm than it will change to "get to 100 than drop"... if you can tune it right it can even turn into a "get to 100 and than evaluate second by second if stay or drop based on circumstances"... this whould probably be the BEST scenario, where the player actualy have to consider his action and not simply do the same thing over and over (while I concur that sitting at 100 all the time make it a poor mechanic the concept of build up, drop to 0 and repeat is not really much more engaging

)
Penril wrote:
- The drawback for being at 100 mechanic should be harsher. Backlash is NOT harsh enough for the fantastic bonuses they gain.
In this regard I seriously thnik that we should drop the concept of "damage as a consequence of high comb/DM" completely... the reason is that self damage is extremely hard (I would rather say impossible) to balance given 2 completely different envorament that are organized WB play and solo/pug play... combustion as it is can be ignored by organized WB but is a pain for Pug/solo... if we make it harsher enough for organized WB to have to actualy take it into account than it will make it impossible for SOLo/Pug to actualy play with it... and while I know that balance here is aimed toward organized WB play there should also be a limit to how much it can affect solo player

especialy if an ALTERNATIVE do exist
with this said I would actualy look at different tipe of drawback;
Damage return, as I explained before is impossible, AoE make it so that you can't get even a 50% damage return without getting killed by a single proc, any lower percentage would turn single target damage return into ignorable as much as current Explosion/Backslash. DoT and channeled skil (and BW/Sorc have plenty of both) make it's implementation even more complicated.
Heal debuff, as pointed out it can be harsh but it can also be completely ignored depending on circumstances (if you'r not targeted than there is no problem with getting less heal ^^).
Wound debuff, sound good at first but with current itemization it can easily turn into a too harsh a punishment if made stackable with other enemy's wound debuff, on the other hand it can turn out incosequantial if it doesn't stack (since Wound debuff are quite common and used in organized play).
AP cost increase, this is what I'm more inclined toward, it is easy to tweak, don't cause the player to drop dead but force him into administrate his resources, it can be made harsh (double or even triple the cost of skill at max Comb/DM) to cicunvent group/self AP regen tactics that should at best ALLEVIATE it and not cancel it (otherwise we go back to explosion/backslash being incosequential in organized WB enviorment) while in Solo/Pug it will simply froce you to drop out of 100 Comb/DM after one or 2 spell (while organized WB should be able to keep at it for a LITTLE longer before getting starved).
It also promote the idea of using High Comb/DM only for BURST damage and not for SUSTAINED damage.
Other, I don't seem to remeber other TYPE of drawback showing up in this tread but I might have missed them ^^'
Penril wrote:
- In return, they should gain something when consuming their mechanic, in order to keep the class as strong/possibly stronger than it currently is.
This has been posted several time in the tread and I'll reiterate once again; Convert some of the current skill for each mastery into combustion/DM consuming skill. AND make them appealing! they don't need to be FINISHER like the SL/Choppa but they should give the player a REASON to use them beside the simple concept of dumping their Comb/DM.
For DD Sear/Gloomburst is quite a good candidate, remove dthe CD and turn it's cost into 25/30 Comb/DM (the cost itself will act as a limit to avoid spam just like it does for SE/SP, it is also a skill that have tactics attached to it (just like SE/SP) that can make it more valuable.
Sorc double gloomburst tactic that inflict a garnted backslash can be changed to double damage but triple DM cost turning it into a mass DM damp. Sear lifetap Tactic could be improved (mybe 100% dmg returned as health) adding a double/triple Combust cost to it to have a similar effect (one gives burst damage at cost of High DM the other give self healing at cost of high Comb)
For DoT it's a bit more complicated since most core ability from this mastery are either under a CD that can't be removed or a too different between the 2 classes... for the BW detonate could be an excellent candidate but Sorc don't have anything equivalent (icy spike if changed to a spammable PBAoE DoT that consume DM will turn in just a mirror of SP). My suggestion for this would be to rework SB/VoTmatbe turning them from simple AP drain into AP leech (returning part of the AP to the caster) while consuming Comb/DM at every tick or all at once at cast (this will also add an AP recovery tool that could be usefull for other changes.. see above about AP increase as consequence of high Comb/DM).
AoE already have SE/SP, those 2 could just be twiked a little to make them more appealing but they already have a good spot so pumping them could not me necessary... need test ^^'
finally, as someone also suggested, beside those that should be "basic tools" (as in avaible to every BW/Sorc) changing the final skill of each mastery into a combustion dump could also be interesting expanding the option and making gathering combustion only to release it in specific moment into an engaging type of game play (don't simply turning it into a drop it every time you get to 100 but rather get to 100 --> TRY to manage it for some time --> use that comb/DM to make something really usefull while also alleviating your high drawback)
this is the best I can think of ^^ hope it can be usefull ^^