The original system that Savage Worlds came from was a pool of dice system and worked well, but the designers felt it could be easier, much easier.
They distilled the pool down to a single dice, with a “wild” dice as the PC/Villain equaliser, to add a curve. At first glance I felt it was too easy and slanted towards weaker characters as the “exploding” mechanism, is rolled was too easy, but I have come to change my mind on that.
The core roll to pass is 4, the dice range from D4 to D12 and the wild dice is D6. Double ones crap out, we will call this a fumble and maximums on the chosen pass dice are an explode (re-roll and add).
The Math;
Rolling an exploding 4 on a d4 is 1/3 easier than rolling a 6 on a d6, but it also has a 75% chance of failing completely (the wild dice is 50%, making it the lucky touch) and the fumble chance is 1 in 24 as opposed to 1 in 36.
Double exploding the d4 is a 1 in 16 chance and a 1 in 64 chance of hitting 12 (2 raises) with a triple explode. the 1d4 and 1d6 has a 25% and 50% chance of success, almost all down to the wild dice and a 1 in 24 chance of fumbling.
An exploding d6 can match the maximum potential of that d4 triple roll with one explode, or a 1 in 36 chance (i.e. almost twice as often), an evens chance of a raise anyway and can reach 3 raises with another roll even without exploding, so more effect and more likely to happen. The 2d6 has two x 50% chance of success. The d6 pair have a 1 in 36 chance of a fumble.
Going to the extreme end, a d12 roll, there is a 1 in 72 fumble chance, a 50% and 75% pass chance, less chance of an exploding roll, but a double raise is possible on a natural roll at 1 in 12 with an explode! The fumble drops to negligible (but not improbable, about double a 2d6 roll), the good gets exponentially better, the wild dice goes from enabler to safety net.
Three raises (sometimes the practical maximum), can be reached by a d4 or d6 in three rolls, d8 and d10 in two rolls, a d12 can get there in two. If you cap explode rolls to a single one for a less swingy game, the d4 can reach one, d6 only two, d8 and above can make three.
I also felt the +1 added after d12 was a patch fix (time the d16 became readily available as a step to the d20), but looking at the math above, it becomes a massive, but controlled boost. The fumble and explode chance of the raw d12 remains, but the success chance goes up to about 50&84% at +1, 50&92% at +2 etc until rolls are only used to determine raises and fumbling fails and often only Strength is boosted anyway, meaning damage dice are boosted.
Simple, clean and as it turns out, effective (seems I substituted a d4 for a d20, my bad).
On the surface, the benefit of the exploding d4 seems too good, but the reality is, it is limited in effect, more likely than not to fail and as increments of 4 are needed for raises, it has to explode every roll to make any real difference.
I would argue, the d4 “benefit” is a good levelling tool at the bottom of the roll tree for those desperate adventurers who “have a crack”, but are of little other real value and the d12 with a “+” bonus is a good way of capping increases.