A lot of those things are myths which continue to thrive. I usually recommend a better hsf regardless of stock and many others do as well. Aftermarket hsf's are more capable of lowering temps and typically use better quality designs and better quality fans which reduces system noise. Under extended use (as in gaming for hours) with the cpu maxed out the stock hsf can barely keep up. Gaming isn't usually considered 'average' use which is what stock coolers are geared toward. Common applications like web browsers, messengers, spreadsheets, word processing programs, viewing videos and so forth places nowhere near the load on a cpu that gaming can and does.
Any case should have adequate airflow, there are decent cases with this capability in all budget ranges. It doesn't take an expensive case to oc. That should be a goal regardless if the machine is a gaming machine or office pc. An oc'd cpu puts out relatively little difference in heat since the vast majority of heat in a gaming rig comes from the gpu (or gpu's if crossfire/sli).
Thermal paste is again personal preference for type, consistency (for application properties). It's already been shown that out of 40+ thermal compounds tested, aside from a couple of no name brand pastes you'd have to go out of your way to even dig up someplace, they all work within 1-3c of one another. To say oh, I'm going to oc, now I have to buy $30 thermal grease has never happened. Thermal compounds with enough for several applications can be had in all varieties and even so called 'performance' pastes run in the $6-10 range. If someone's spending $350 on a gpu they really shouldn't cry over $8 thermal paste.
Anything is a risk and there can be risk's when oc'ing. It takes effort, for users who don't feel like putting in the effort to do it correctly then oc'ing isn't for them. Oc'ing anymore is far safer than it has been in the past. The k series cpu's that don't oc well are far fewer than those that give performance consistent with averages.
People checking the benchmarks of current processors comparing today's cpu with today's game oc'd vs stock are in a sense missing the point. Since oc'ing can help an aging cpu handle new games with new gpu's as they become available. Meaning no, you won't see much difference now. Give it a few years with more powerful gpu's than exist today or multiple newer gpu's than exist today playing games that are more demanding than they are today with no benchmarks to try and justify it since there are no bench's from the future around.. and yes, the oc'd cpu will have a longer useful life.
Think of it like a 2500k. The performance difference between the 2500k stock and 4690k stock can be negated by overclocking the older 2500k. Here's a perfect example of what I was referring to. Looking at skyrim and the 4690k, fairly recent game and current cpu. Overclocked, the 4690k gains 7fps average and 5fps min framerates over the stock 4690k. Not a huge deal sure. Now looking at the i5 2500k, an older cpu on a more modern game. When overclocked, the 2500k is able to gain 21fps average and 19fps min framerates above what it was capable of at stock. Gaining 20fps when playing current games on an aging cpu by overclocking it is hardly 'no performance gain'. Where do you think the 4690k will be in 4yrs? In the same position the 2500k is now.
So say you spent even $100 more for the ability to overclock that 2500k when you bought it new. Additional for the cpu, better motherboard, better cooler. After 4yrs, that's $25 a year or $2 a month. In order to get the same level of performance as an oc'd 2500k, you'd be buying a 4690k for roughly $220 plus a new motherboard if you'd gone with the 2500 non k. To save $100 (or less) up front it would end up costing the thrifty user $350 to play the current games at the same level of performance the overclocked user is still playing at.
Same thing in shogun 2.
http://www.bit-tech.net/hardware/2014/07/03/intel-core-i5-4690k-review/5