Long answer..... Not a lot.
Not a lot.
Think cars... the difference between a midsize Chevy 4 passenger sedan and a midsize Ford 4 passenger sedan.
Not a lot of difference. Yes there are styling differences, some times quite dramatic styling differences. Maybe the Ford has 2 more horsepower, but the Chevy has 5lbs more torque. Then again, the Ford's torque peak is 500 rpm lower than the Chevy, but, wait, the Chevy gets 1 mpg better than the Ford. Ooops, just forgot, the Chevy trunk is 1 cubic foot bigger than the Ford, but the Ford will hold 5 suitcases, and the Chevy will only 4 suitcases and two overnight bags.
Think I am joking?
If Chevy adds pink fur lined heated coffee mug holders as standard equipment, next year Ford will offer green felt lined heated coffee cup holders as standard equipment.
In other words, there is not a lot of real differences, except at the introduction of the model year, and then, the differences are often really minor.
Comparing a companies $800 receiver versus the other companies $795 receiver is about as meaningful as comparing the cup holders between automobiles.
The differences in Audio can be just as minor......
.....Denon uses a detached mic for their setup. HK uses a port on the remote to serve as the mic. Is that a dramatic difference? Want to compare power ratings? HK is considered to be more accurate in their power rating, yet Yamaha and Denon in an independent testing were closer to the rated power output than HK. The S/N ratio of the Denon and Yamaha measures better than the Onkyo which measures better than the HK, yet my ears say the HK is as quite as the Denon and quieter than the Yamaha. By the way, once we get to a near flagship model the background noise level is so minor that in order to hear the noise on my Denon, I must turn the volume to well above the 0db setting with zero signal being fed to the receiver ( in other words I am applifying the background noise of the receiver to astronomical levels before it can be heard). Just as a note, 0db is a volume level that is much higher than the ear bleed volume in my room which is about the 10db setting.
Maybe, maybe, Yamaha and Denon are more cutting edge than the other brands. Maybe. It is hard to judge a product when we are comparing a minor circuit, or whether a brand has a better surround sound feature. And obviously, what ever feature Denon adds this week, Yamaha will add next month, and somebody else will have next quarter. Then again, Samsung just came out with a super receiver that seems state of the art. See how it is hard to write absolute rules about the brands?
There are differences in reliability, just like there are differences in the reliability of cars. You ask my independant auto mechanic, and he dislikes Chrysler products. Maybe he just had a bad experience working on one car. Maybe the Chrysler products are less reliable. Maybe he just doesn't like the color red they use one their products. Maybe Ford gives him a kickback to bash Chrysler.
Audio equipment is just as fickle in reliability. I have read absolute bashing of a brand for reliability. Are those posts meaningful? One, two, or even 100 hundred such posts are meaningless if the manufacturer has 100,000 extremely happy customers with units that have never needed repair. Real world data indicates the life expectancy of audio equipment to be 10 or even 20 years, IF the unit survives the first 30 to 90 days of operation, AND early failure is often a byproduct of shipping and handling rather than a design or manufacturing problem.
(think about it, how can Yamaha's Burr-Brown DAC's be less reliable than Denon's Burr-Brown DAC's? Maybe, Yamaha received a bad batch. Maybe. Then again, Denon may have received the bad batch of DAC's {failure rate of 1 in 1,000,000} and Yamaha got the standard DAC's {failure rate of 1 in 250,000,000}, and the Denon owners, all three of them world wide, complained about their DAC failure, while the two Yamaha owners never publically complained.
Manufacturering location. Everybody uses third world countries to manufacture their entry level equipment. Sometimes the electronics plant is making several brands of equipment on the same assembly line. Some companies have all of their equipment made in those countries. Others uses Japan to manufacturer their flagship and near flagship models. Is one location better than another?
One of my electronic manufacturers (not audio) inspected a plant in Mexico that was making a circuit component. The electronic manufacturer had always purchased the "red" (aka "premium") circuits because the specs were far superior to the "blue" (aka "economy") circuits and the price difference was only 20%. Well, much to his surprise the blue and reds were made on the same assembly line. At the end of the line, a gentleman grabbed a circuit with his left hand and placed it in the "red" box. The same gentleman grabbed a circuit with his right hand and placed it in the "blue" box. Because the gentleman was right handed, his production of "blue" circuits was slightly higher than his production of "red" circuits. Needless to say, the manufacturer now buys the "blue" circuits and saves 20%.
So what differences do exist? Cosmetics. Ergonomics. Trivial featues. And remember, what you like, is not what I like.