Well, I think the change would have 2 overall effects on the game.
1) It would discourage 6 dicing a spell. 6 dicing a spell for IF removes tactics from the game by preventing your opponent from dispelling. The goal isn't to necessarily make nuke spells impossible to cast, but rather to punish the player who tries to force through a nuke spell with IF. Throwing less than 6 dice decreases the change of an IF pretty dramatically, but with a min casting value of 18 for dwellers or okkams, 15 for sun and final trans, its possible to reach the min value with a 4 dice cast (but its still difficult with a LV 1). Meanwhile, if you by chance miscast with 4 dice, there is only a 1/12 chance of losing your mage (which is the same as the current probability) but 2/3 chance of not getting to cast that spell again. Punishing, but not necessarily devastating. The key point, I think, is that throwing only 4 or 5 dice at an attempt makes it much less likely to cast with IF, and therefore gives your opponent a chance to dispel.
This, then, makes the magic phase much more tactical. Can you get your opponent to waste dispel dice on another spell then get off your nuke? Or otherwise, can you cast it and force your opponent to dispel, only to then cast another 3 smaller spells when he uses up all DD on the first spell you cast? It would make it more of a balancing act, rather than a case of "there's nothing you can do, now remove half your unit."
Also, the unboosted spells often have shorter range, making it less likely to go off turn 1 (nothing is worse than losing your general and half his unit before you even get to move). You need to position much more carefully when you are trying to cast a shorter range spell with a potentially fragile mage. So that, once again, makes the game more tactical.
2) It makes it so it is STILL POSSIBLE to force through a spell with 6 dice. Unlike with ETC where 6 dicing a nuke is illegal, its still legal here, but very punishing. You have it as an option if you are in a situation where its worth losing your mage to get the spell off. If your opponent put himself in a situation where forcing through a nuke spell can win you the game, then you are free to throw 6 dice at it and capitalize on his (or her) mistake. But you then must be aware that you will likely only get 1 shot at that IF nuke, so it better be worth it.
These two points bring it to a place where its difficult for your opponent to cast spells that you have no chance at dispelling. If you take a deathstar, expect someone to try to force through a big spell on you because its worth the risk. If, however, you spread your points and your characters, giving them no valuable targets, then your opponent will be unlikely to throw 6 at a spell because it won't be rewarding enough to risk his LV 4. Much more tactical of a phase IMO.
@najo I like your changes (except for the first change), but I don't think they necessarily balance deathstars and magic.
1) Allowing ward saves on nukes would just make MR items mandatory in deathstars. It would make the nukes less powerful, but it would also make deathstars better. In fact, it would reward stacking characters in the same unit, because that way one MR item carries over to all the characters. There would be no risk to having all characters in the same unit, as they would all get the benefit of MR.
If they also changed MR to give extra DD (to prevent ward saves in big units), well then the game mechanics just start to reward people even more for 6-dicing spells again as MR would then be useless. In either case, whether MR gives ward saves, or counts as DD, it changes the game to make it less tactical (reward deathstars or reward 6-dicing).
2) I love this idea, and I think they should implement it. I would love for flanks and rear charges to actually matter! I don't like that deep units can march into enemy territory and give up flank charges only to say "Oh, I'm steadfast!" Flank protection should matter more. I also think that steadfast should work more like 7th ed stubborn. Ie, when you lose combat, you either use the OWN UNIT'S unmodified LD, or the general's modified LD, whichever is higher.
Maybe they should even make being charged in the flank count for -1 to enemy's LD instead of +1 CR (and rear charge counts for -2 LD). This would make being flank charged by fear causing enemy more devastating, and would circumvent steadfast/stubborn. It wouldn't matter for calculating who wins but just be a straight up negative to enemy's LD. It would cause the same net effect as it does now if you break steadfast, but it would also mean much more if you flank charge but don't disrupt.
This doesn't fix deathstars, though, since most deathstars are stubborn anyway thanks to the crown, and therefore don't rely on steadfast. I do think they should remove the crown from the game, as cheap stubborn just makes the game less tactical. But even if they did, most good deathstars also don't lose combat, so that isn't going to work to discourage them.
3) 100% agree here. I'm not a fan of pts denial tactics.