Like the title says, I'm just a little bit curious about why Riot made the decisions they did around the cooldown reduction stat, and whether you forumites (or even a Rioter) would agree with that choice. First, I'll do a little groundwork.
Reducing your cooldowns is a cool and interesting part of the game, but it naturally has a big risk - if cooldowns get too low, people can get CC-locked, have far too high uptimes on strong buffs, or just spam out so much damage that nothing can survive. Obviously, you need to stop someone from stacking enough to get no cooldown whatsoever. The way I see it, there are three clear ways to avoid this.
1. Multiplicative scaling. Like with percentage Armor Penetration/Reduction, you could stack the cooldown reduction effects by multiplying them. That means if you buy an item with 10% Cooldown Reduction, all your cooldowns will go down by 10% of their current amount, whether you have no CDR or 5 other items. This is very simple, and fits the pattern laid out by the percentage defense reduction effects. It also means that the only way to get no cooldown is with a single 100% CDR buff. Of course, if you stack a lot of CDR items, you can get very high uptimes - six 20% CDR buffs is equivalent to a single 73.8% CDR buff - almost enough for a TF with very good reflexes and luck to chain-stun with Gold Cards. Certainly enough to make teamfights a matter of who has the more disruptive tank. It also makes large buffs much more valuable than small ones, even before slot issues take effect - a 20% buff is 1% more CDR than two 10% ones, which might be a little counterintuitive.
2. Increased recharge rate. Instead of selling a reduction in your cooldowns, sell items that give a percentage increase to your maximum rate of fire, much like Attack Speed does. These would stack additively, but each point of CDR would have the same value. For example, Veigar can fire off 0.25 Baleful Strikes per second at rank 5. A 50% increase in recharge rate would be 0.375 per second, or a cooldown of 2.67 seconds - which reduces the base 4 second cooldown by 33% . On the upside, this both prevents catastrophic scaling as cooldowns approach zero (because the amount of recharge needed to gain 1% of cooldowns is increasing at the same rate), and lets each point of the stat be worth the same amount of DPS, all else being equal. Of course, this is going to be a flat numbers nerf to all the CDR numbers you're used to - where 100% CDR means no cooldown, +100% recharge rate means 50% CDR, as you can use the ability twice as often as normal.
A few quick conversions for those not familiar with the math:
10% CDR = 11% Recharge
20% CDR = 25% Recharge
30% CDR = 42% Recharge
40% CDR = 66% Recharge
Formula: R= (1 - 1/(100 - C))*100, where C is cooldown reduction and R is recharge bonus. It's closely related to the armor/MR calculations for turning the number in the display into a percentage damage reduction, and it serves the same purpose - to make one point of stat give the same benefit at all levels.
This means that we can do away with the hard cap, get easily costed stats, and if you have a reason to, then you *can* stack recharge all day and go to town - just like you could stack damage, or resistances, or health.
3. Additive stacking with cap. What the game actually does. Working out your CDR is quick and easy because of the additive stacking, but since each point of CDR is worth progressively more (as above), there has to be a point where we cut it off or else it simply becomes better than everything else. In this game, it's 40% - which is good, as past 40% CDR starts climbing in value really quickly. The problems with this are that CDR becomes a binary stat - either you shoot for the cap with no overshoot, or you really don't care, because the first 10% is much, much worse than the last 10%; that the cap is not always clearly available to players (nowhere in the documentation does it acutally tell you that 40% is the magic number - you have to look at the champion stats page); and it introduces a hard cap, something which is not used elsewhere in the game. No other stat has a clear "this much and no more" point.
I'm kind of curious why people think that this was chosen, when I just outlined two other ways it could be avoided. Also, I'd like to know which of these options (or another if you think I've missed something) you would choose if you were designing LoL from scratch.