This post is about the issues I see with cooldown reduction, some mathematical breakdowns, and a possible solution I am putting forward.

Problem: The 40% cap doesn't interact well with Blue Buff, champion abilities, and has strange scaling where it has increased returns the more you get.

Notes:
-The combination of Blue Buff and 15% CDR reaching the 40% cap makes building more than 15% CDR wasteful, when CDR should be a useful and stackable way to increase caster spell DPS the same way Attack Speed increases AD DPS.
-CDR scales strangely: the first 10% you buy is worth much less than the last 10% you buy. This makes only getting 1 CDR item extremely wasteful if you arent a blue-buff-getter.
-The strange scaling, cap, and blue buff interaction make CDR a hard to manage inconsistent stat, when it could be a fun stackable way to increase caster DPS and diversify itemization.

Explanation of CDR Scaling:
The amount of CDR you have IS NOT a representation of how much extra damage output you will have over time with spells. 40% Faster Cast does not equal 40% more damage. Also, the last 10 CDR increase from 30-40% gives you a lot more than the first 10 CDR increase between 0-10%.

Here are the numbers:
10% CDR: 10/9 +11% Faster Cast Rate/Spell DPS
20% CDR: 10/8 +25% Faster Cast Rate/Spell DPS
30% CDR: 10/7 +42% Faster Cast Rate/Spell DPS
40% CDR: 10/6 +66% Faster Cast Rate/Spell DPS

note:
The first 10% adds +11% Faster Cast, while the last 10% adds 24% Faster Cast. Therefore, the last 10% you buy is worth more than 2x more than the first 10%.

An extreme example would be the fact that 90% CDR is actually +1,000% spell DPS, and 99% CDR is +10,000% spell DPS.

Solution: Cooldown Reduction could be replaced by Faster Cast Rate, which would scale on cooldowns the same way attack speed scales on auto attacks, linearly.

Calculation:
Cooldown = BaseCooldown/(1+FasterCastRate)

FASTER CAST RATE STATS EXAMPLE:
Faster Cast Rate versus a 10 second cooldown and equivalent CDR

0% FCR = 10 second cooldown = 0% CDR
20% FCR = 8.3 second cooldown = 17% CDR
40% FCR = 7.1 second cooldown = 29% CDR
60% FCR = 6.3 second cooldown = 37% CDR
80% FCR = 5.6 second cooldown = 44% CDR
100% FCR = 5 second cooldown = 50% CDR

The amount of FCR that is given on items would have to be adjusted to fit this new calculation, of course.

Benefits of this change:
-Some items could be given high FCR numbers which would be flavorful and interesting, such as a high-end caster item balanced around giving a huge +60% Faster Cast Rate bonus.
-Blue Buff and Blue Potion would stack safely with other Faster Cast effects the same way Attack Speed bonuses stack with other Attack Speed Bonuses.
-Once the numbers were adjusted, the new linear calculation would make having only one source of faster casting worth it, while currently only having one source of CDR is inefficient.
-Itemization would be made more interesting because you could stack more CDR items, and then pick up blue buff and and blue potion to stack huge amounts of FCR in a safe linearly stacking non-broken way.
-CC abilities would not become too powerful given the fact that the stun durations would always be far shorter than the remaining cooldown even with huge amounts of FCR. Example: a 1.5 second stun on a 14 second cooldown with 120% FCR would be 1.5 second stun with a 6.3 second cooldown. FCR could be capped at 120% or 150% as well, which would maintain a large enough amount of non-cc time between ability casts.
-Characters with built in CDR effects have their options limited because of the 40% CDR cap. Ryze can only build 30% CDR while other characters are allowed to build 40%, which limits his build choices and is just not as fun. With the FCR system, characters with built in FCR would be able to combo their FCR abilities with more FCR.
-ionian boots would be useful

edit:
What do you guys think? Please ask any questions you have and I will try to respond to them.

Please don't respond with: "NO THATS DUMB X WILL BE BROKEN WITH X." Please explain what problems you have and why you think they are a problem so that we can discuss solutions if the problem does exist.

Here is another possibly easier way to display the information comparing the current system with the proposed system:
CDR:
0% : 10 second cooldown casts 100 times every 1000 seconds
10%: 9 second cooldown casts 111 times every 1000 seconds
20%: 8 second cooldown casts 125 times every 1000 seconds
30%: 7 second cooldown casts 143 times every 1000 seconds
40%: 6 second cooldown casts 166 times every 1000 seconds

Faster Cast Rate:
0%: 10 second cooldown casts 100 times every 1000 seconds
10%: 9.09 second cooldown casts 110 times every 1000 seconds
20%: 8.33 second cooldown casts 120 times every 1000 seconds
30%: 7.69 second cooldown casts 130 times every 1000 seconds
40%: 7.14 second cooldown casts 140 times every 1000 seconds
50%: 6.66 second cooldown casts 150 times every 1000 seconds
60%: 6.25 second cooldown casts 160 times every 1000 seconds
70%: 7.14 second cooldown casts 170 times every 1000 seconds

Another good way of revealing the problem:

It is extremely odd that Blue Buff adds more spells cast to someone who has more spells cast than it does to someone who has less spells cast.

This is a comparison of what happens when blue buff is added on top of 0% CDR, and then added on top of 20% CDR bringing someone to 40%.

If you can cast10 spells in a given amount of time, blue buff adds about 2 extra. 10+2
If you can cast 10+2 spells cast from already having 20% CDR, blue buff adds about 4 extra 10+2+4

(it is really:
10+20% = 10+2.5
10+20%+20% = 10+2.5+4.1
but I wanted to make it clearer so I rounded to whole numbers)

sounds almost exactly like a thread i posted yesterday... or rather... 6 hours before you posted this one.

While admirable, cd isn't one of those necessarily out of whack, or grand fathered stats as we found when riot introduced tenacity to cover the CC problem. I believe at best they would just increase the item premiums which just force you to build otherwise. And if anything it might even be a side swipe to get blue buff out of the same pattern its been in for some new playstyle changes.

However its been ages since this stat has been talked about considering ad stats have been more in the favored light. Might be nice to get a refresh on how they perceive it now.

Fun read though.

Bumb for tracking purposes

The main thing that has to be considered is interaction between champions at high amounts of CDR. For example, if you have shen, alistar, and cho'gath on the same team all with 150 FCR (60% CDR), shen's taunt is an AoE 1.5 second taunt with a 3.2 second cooldown (8*(1 - .6)), alistar' pulverize is a 1.5 second AoE stun with a 5.2 second cooldown, and cho'gath's rupture is a 1 second AoE knockup with a 3.6 second cooldown. If these three care chained together perfectly, you can actually get a shadow dash -> rupture -> pulverize -> shadow dash -> rupture uninterrupted for a whopping 6.5 seconds of hard CC on potentially a whole team, with shadow dash coming back up in .7 seconds, pulverize coming back up in 1.1 seconds, and rupture coming back up in 2.6 seconds.

Add in janna's howling gale, a .9 second AoE knock-up on a 4 second cooldown, but since it could potentially be charged up while the other CC are holding the enemies in place, that can make it a 1.1 second knock-up. By chaining those 4 abilities together one could potentially infinite CC lock a group of enemies. Now of course this can be changed when tenacity comes into play, but three out of four of these are knock-ups, which are not reduced by tenacity.

The system would have to be balanced such that a champion achieving any more than 100 FCR is nearly impossible.

Alternatively, the cooldown on shadow dash and rupture need to be increased.

Also veigar's event horizon.

I guess the OP forgot about this thread, or at least decided to forget about it after seeing that no one had replied to it for a whole 16 hours since it was posted... I kind of want riot to see at least one of the threads on this issue so they can get their **** together.