I've been meaning to make a post about this, albeit a bit more in-detail. However, this will be just fine! I got a banging headache and fever so hopefully I don't mess up this train of thought.
So, as some people have previously suggested, Diablo seems a candidate to also adopt a ppm (proc per minute) mechanic. If you're not familiar with the WoW mechanic that is ppm and its inherent flaws when using it in a combat system that is WoW melee, here's a short description:
The % chance to proc something in a ppm system is not flat, it's fluctuating with the base attack speed of your weapon in order to maintain a decently even ppm among weapon speeds. As such, faster weapons have a lower chance to proc per swing than slower weapons.
WoW PPM mechanics description
In WoW there is a flaw with this setup, the ppm mechanics are assuming that white attacks only are dealt. WoW has "another" layer of attacks (special attacks, the abilities you cast yourself) on top of the auto attacks that aren't as much dependant on weapon speeds (there are exceptions such as rogues' combat potency), and therefore the amount of special attacks dealt stays (almost) the same among weapon speeds. These special attacks can in most cases also trigger procs, with the same proc rate as the corresponding white attacks. This skews ppm mechanics to favor slower weapons to proc more, which might seem like a fallacy.
If you have an issue seeing why, we'll take an extreme example. Let's say we have a 10 speed weapon (ie 10 second swingtimer, or 0,1 APS) and let's say this equals to a 100% proc rate. And in the other hand we have a 1 speed weapon, which let's say equals to a 10% proc rate. The slow weapon will autoattack to a ratio of 1:10, but each proc is guaranteed. On average, they will both proc the same amount from white hits per minute. Now, let's add special attacks. WoW has a base gcd of 1,5 sec for most classes (not always the case, but let's use that). And let's assume they have a resource other than rage, such as energy or runic power. During these 10 seconds, both will deal the same amount of special attacks, but the slower weapons will proc each attack, whereas the fast won't, resulting in a much higher number of procs for the slower weapon.
Enter Diablo 3.
There are no auto-attacks at all, the weapon speed differencies equals to an increase of number of attacks dealt by a perfect ratio of aps 1: aps 2. This is the perfect system to use ppm based proc mechanics for, it will provide a much more even number of procs amongst the weapon speeds.
I simplified ppm a bit, last I checked WoW used a formula consisting of this:
Proc % Swing = Base WS * PPM / 60
The PPM is not a known variable, this has to be derived purely from testing and extracting procs from large samples of data. This would obviously prove difficult for three main reasons: there are no dolls, several effects are chance on crit, and we don't have any combat log.
However, that's all fine in a sense, because they're going to become very even in the Diablo universe. In fact, they'll be perfectly equal as long as the IAS remains the same (if it uses base weapon speed rather than hasted attack speed). I'll show you a few different weapon speeds and their proc rates. The proc rates all uses the formula of WS * PPM / 60. In this case the PPM is an arbitrary number of 6 ppm.
WS % Swing APS*Proc%*60
1,1 ~ 9,1% 6 ppm
1,2 ~ 8,3% 6 ppm
1,3 ~ 7,7% 6 ppm
1,4 ~ 7,1% 6 ppm
1,5 ~ 6,7% 6 ppm
I should note that there are exceptions to this. WW seems to have a somewhat set APS which would highly favor slower weapons for both damage and procs. Abilities such as Ancient Spear also seem to use a set APS of 1.2 (or thereabout), which again would favor slower weapons.
Another important note is that this proc % swing and formula assumes non-crit procs. For the crit-based procs (of which we have several) another variable must be added to offset the increased number of crits/second for faster weapons even further (if you wanted a perfectly even number of actual ppm before IAS). I might be wrong here, my head's a little wonky at the moment.
Enter IAS effects.
These will be the determining factor of which procs more attacks. The PPM formula uses only the base weapon speed, and as such, more attacks equals to more procs in the same period of time. There is of course the possibility that IAS effects does fade off the proc % swing as well, but in this particular case I doubt it. There would be no increased return of procs with increased weapon speeds at all, and that appears to go against Bashiok and Jay Wilsons previous quotes that dual-wielding will be the proc machine of the gods. They might have overstated the procs of dual-wielding possibly (if d3 uses a ppm system), and it's possible that they still don't weigh the ppm to be exactly equal, but just to close the gap somewhat and so on.
The biggest reason why I think Diablo 3 is going to use a ppm mechanic is simply down to the tooltips, and that they have similar systems in place for certain spells and %wpn dmg, where the weapon speed will determine your %wpn dmg. We get to know no information other than that there's a chance to proc something. This is exactly what we're seeing in WoW when ppm mechanics are involved.
I reiterate, there are many nuances and possibilities to a ppm system. If it uses base weapon speeds, IAS effects will linearly increase the amount of procs, which favors dual-wielding builds in terms of number of procs due to the possibility of having slightly higher IAS in similar builds.