The complexity in 3e maths was not due to the classes giving different BAB and save progressions - these were all clearly laid out in the class tables, and the multiclassing rules were trivial (just add the bonuses). There were problems with classes giving different progressions, particularly at high levels, but complexity wasn't one of them.
Rather, the complexity in the maths in 3e was the multitude of feats, powers, spells, magic items, and other special cases that applied modifier on top of modifier. The stacking rules really didn't help with this, because although they were simple in concept (bonuses of the same name don't stack), in practice the huge number of bonus types created big problems. (Quick: I have Protection from Evil and Protection from Law active, a Ring of Protection +1, I am hasted, am wearing Bracers of Armour, have been polymorphed into a Troglodyte, am wearing an Amulet of Natural Armour, Full Plate +3, and a Shield +2, and my Dex is 12. I'm being attacked by a Slaad (Chaotic Neutral). What's my armour class?)
Unfortunately, 4e has moved all the differentiation between characters into special cases, rather than clearly spelling them out up-front. That Rogue class gives some (as yet unknown) bonus to Reflex defense, but it's not presented in the class table - I have to remember it.
There may well be simplifications in the maths in 4e. And there are certainly good reasons to go with fixed progressions across all classes, especially if they actually intend people to play at Epic levels. But the fixed progression is not, in itself, a simplifying factor - we're just trading twelve tables for one, at the expense of twelve different sets of special case modifiers.