Scaling has got to be one of the one misunderstood and abused topics in game design. Scales badly is a, if not the, most common way of saying that something is bad. Despite the whining though scaling is actually a really critical factor in keeping classes and skills balanced long term. I have been doing some looking into how classes can ramp up their damage in D3 and found some disturbing issues. The major problem is the discrepancy between class resource mechanics work with increased attack speed.
Some classes like Monk, Barbarian, Demon hunter (thugs) generate resources from some attacks and spend them with others. Increased attack speed (IAS) means that they generate more resources and spend more resources - 50% IAS certainly doesn't increase resource generation by 50% because there are static sources but it helps a lot. Wizards and Witch Doctors (casters) have resource pools that regenerate on their own and they don't get resources back by using attacks in any significant way. This means that IAS is somewhere between nearly useless and not so great because it doesn't let them attack more often since they are resource limited already. I can't attack extra times if I am already out of mana!
The reason this is such an issue is that IAS is by far the most powerful and easiest way to increase damage output at high gear levels. Once an item has your prime stat on it you can't just stack more prime stat - you need to go to other stats and IAS is the best. You can, if you stack huge amounts of +crit chance and +crit damage increase, get decent damage returns from crit but IAS is cheap and doesn't require other stats to be effective. It is pretty easy to get 60% IAS on gear without much investment and that is a really big problem when some classes get 50% more damage out of it and some get 0.
This pretty much revolves around the different resource systems not being similarly designed. Witch Doctors actually have to spend mana even for their very cheap spells so it isn't sensible to have more than 1 attack spell. If your attack spell is cheap you can't add an expensive spell because you can't afford to cast it and if your attack spell is expensive you can only cast it if you don't have another attack spells in your build! In my Splinter build for my Witch Doctor I cast just the cheapest spell available and even if I wanted to I couldn't put an expensive spell into the build - I can't cast it. Wizards aren't quite as badly off because they can actually have a cheap spell and an expensive spell in their build and use both so they can make *some* use of IAS but it is still much weaker than thug classes get.
There are other scaling issues that D3 handled really well however. In D2 spell damage scaling and weapon scaling was nutty and made no sense but those are handled very smoothly in D3. Crit scaling seems quite reasonable as there is some strategy in terms of when to go for stacking crit effects and when not to. Prime stat damage scaling work well and weapon scaling works well - IAS is the standout problem and really it just points to a major flaw in resource system design for Witch Doctors. Witch Doctors need their mana regen baseline massively buffed, Vision Quest nerfed or changed and probably need their cheap spells to be actually free. Until that happens they are going to be stuck in really one dimensional builds with almost no options.