My ideal level of game-mechanics crunch is a little less than normal Savage Worlds, say 80% to 90% of SW's complexity, whereas Deadlands Reloaded runs about 115% of Savage Worlds, and F# is (to it's credit) only about 10-15% as crunchy as Savage Worlds.and thought "how annoying. There's got to be a better way to express that concept!"
After some thought, I think I hit on it - how about if we express the complexity of game systems as die types? We'll use a "c" instead of a "d" so as to reduce confusion. Here's the scale I'm using:
c20: We start with c20, it establishes our first and most basic baseline for comparison. D&D 2.0, 3.0 and 3.5, along with the majority of their variants would be c20s. It's simple: d20 equals c20. If it's less complex than D&D 3.X, you bump it down a die type, if it's more complex, you go up a step or two (see below). 3.X, given enough player-accessible source books (especially mixed 3rd-party sources) can easily creep up above a c20 as well.
Another classic c20 game would be CP2020 - Cyberpunk doesn't have the tricky combat options of 3.X D&D, but makes up for it with the way it handles armor and damage, the enormous list of redundant skills, and the way cyberware totally transforms characters.
c12: A game like Savage Worlds is a c12. It's got some complexity, but streamlines a number of elements to simplify and speed up things better than c20 style game. A lot of generic systems fall into this category - they're not as detailed as D&D, but they still have fiddly bits. Many of them allow for a good deal of variety.
Old World of Darkness is a c12. It's almost light enough to be a c10, but the sliding success scale just nudges it up. Stick to just one type of supernatural, especially if it's just Vampires, and you're probably safe calling it a c10. Most of the complexity comes from mixing the streams, Mage, or that Combat book. Any of those make its a solid c12, maybe even a c14 (see below).
Other c12 examples would be Feng Shui (where the shot clock complicates some otherwise simple rules) and Rune (where the rigors of scenario design and need for player to know the game well enough to GM it bumps it up from the c8 the actual rules would have otherwise been).
c10: Mind's Eye Theatre (at least, the old Rock/Paper/Scissors version) is about here, but again only if you're not mixing your Supernatural types. It's core mechanics are fast and simple, but there's still a some fiddly bits, and a plenty of powers to memorize.
Old School D&D is probably in this category, too, but for the inverse reason - it's rules relied on silly charts, but were pretty darned light when you weren't looking at a chart. It's a c10 that feels like a c8 if you're not the GM or a spell-caster.
c8: Fate, Dogs In The Vineyard, Trail of Cthulhu, and the like. These are games that still have some granularity to their core mechanic and some modifiers to remember, but actively discourage miniatures, and tend to not have many spell descriptions to memorize, nor corner-case rules to look up. A game like this probably won't have an encumberance chart, instead abstracting weight-lifting to a strength roll, GM fiat, or common sense. What complexity is left here is most likely focused into character creation and a skill list. If a more experienced player walks you through the first session, you'll be running at full speed by the second one.
c6: We're deep into Indie-RPG "Hippy Game" territory, with things like InSpectres or Universalis (or Risus when you're not using those optional rules that most GMs skip). At this level, a game has only one or two core mechanics, and very few bells and whistles. There's certainly no spell lists, and not more than one or two charts. There's either no skills, or you can make your own. You can make a strategic mistake in a game like this, but not a tactical one. c6s are pretty simple, and you can usually get a handle on the whole game in a single session even if it's your first play. Fudge is probably a c6, though the use of words instead of numbers can make it feel like a c8 on your first couple play-throughs.
A Trail of Cthulhu game set in the "Purist" idiom, where combat was always a bad idea, and neither Stability or Sanity could be recovered would probably be a c6 rather than a c8.
c4: The epitome of elegance, such as Wushu, F#, or Risus at it's most simple version. Players define their own skills, or traits, or whatever you call it, and everyone starts out mechanically identical, with only flavor distinctions. One type of die roll solves everything, and modifiers are off the cuff. The rules fit on a single page, or close to it. If it got any less complex, you'd be flipping a coin, not rolling a die. ;)
Oddball dice: There are, of course, many dice beyond those found in a traditional 7-set, and I'd be remiss to leave them out:
If you're running a D&D game with a lot of houserules or allowing just about any published OGL sourcebook, then you're looking at something a bit more complex than a mere c20. You've bumped up to one of those odd-ball dice, a c24 or a c30. Warhammer Fantasy is also probably a c24, maybe a c30 if the GM relies on the random charts a lot or really pushes the Chaos themes.
It's a rare game, and again probably chart-filled, that gets above that, but I've seen (and will never play) a couple of games that looked like they might push towards a c100.
There's also few games out there that are like the tens-place d10 (numbered 00, 10, 20, 30, etc.) These are games with much lighter abstract mechanic, but a huge powers list, or the ability to make your own spells on the fly. In other words, simple games till you look at all the weird corner cases that players can push the system towards. That's my impression of Ars Magica and Mage: The Ascension, but I haven't played enough of either to really speak knowledgeably.
Between the c12 and c20 there's a couple of other weird dice as well, though most games stick heavily to the more common complexities. Just the same, I've seen some c14s and c16s out there. Exalted falls into that territory, as does Deadlands Reloaded. I think D&D 4th falls in this range as well, but I haven't played it enough to say for sure.
Scion was a c16 that'd had a corner filed down, so it rolled kinda funny and made the GM nervous.
Continuum was one of those double dice c10s. A hollow translucent d10 with a smaller opaque d10 inside it. It's skill and combat system was a little complicated, and the time-combat system was also a little complicated. Either on it's own would have been fast and easy, but trying to parse both simultaneously was a bit of a challenge.
And then there's the coin flips and the rock/paper/scissors affairs, the ultra-light c2s. Games that really have no mechanics, and are just collaborative storytelling experiments. Games like Amber, that hide the character sheets after the first session, and only let the GM know how the rules work.
My tastes typically run about a c10, but you'll note that I offer very few examples of that complexity level. I'm just a crabby old grognard. Most games are either too complicated or not complicated enough. I like using miniatures when it's appropriate, and I want the PCs to each have their own special niche that no one else can fill. Even for the games I rated at a c10, I wasn't completely happy with. I've GM'd things all the way up to a c24 (7th Sea with a bunch of house rules, Rune with encounter design included, D&D with a lot of sourcebooks being utilized, etc), and also run extended (and successful) campaigns at complexity c2 (Amber and Everway). If a game is too complicated, I'll get sick of dealing with it, but the opposite has never really happened.