There are a few things that make a commercial espresso machine stand out from a prosumer, which makes it stand out from a consumer espresso machine. We'll start with the consumer espresso machines and work our way up to the engineering marvel that is the Rancilio Epoca. At the end of the day, an espresso machine does two things: Brew Espresso and Steam milk.
Standard consumer espresso machines (sub $600) are single boiler machines. They share a single boiler, essentially a store for hot water, for both brewing espresso and steaming milk. Brewing espresso involves injecting 200F water through coffee grinds fine enough to build up 10 PSI of pressure. The water is pushed through the coffee grinds to make delicious espresso. Steaming milk involves generating steam from water in the boiler and shooting it out of a steam wand. Every feature in an espresso machine helps one or both of these functions.
In the sub $600 range, you're really looking at two factors. First is the turnaround time between brewing espresso and steaming milk. Since there is only one boiler, if you switch from brewing espresso to steaming milk, it takes some time for the boiler to generate steam. If you generate steam then brew espresso, it takes some time to cool the water in the boiler down to condense steam back into water. Swapping between the two functions usually takes a few minutes. Second is how much steam / espresso can be brewed continuously before a drop in temperature or pressure occurs. A perfect espresso is 200F for all 20 - 25 seconds of the shot. Cheaper machines drop temperature as hot water leaves the boiler or produce inadequate steam pressure to create latte art as steam is being used.
As you go up in price in the single boiler arena, the amount of steam that is generated or water the boiler can hold increases. Latte art quality milk milk becomes feasible. Drops in pressure as espresso is brewed become smaller or non-existent. The best single boiler machine money can buy is the Rancilio Silvia.
Between $600 - $900 there is a real gap. At the $900 - $2000 range, you're entering the prosumer market. The biggest feature you're getting is a heat exchanger (HX). A single boiler machine with an HX allows you to simultaneously brew coffee and steam milk at the same time. Most of these machines also have a large enough boiler capacity that you can steam milk and coffee for over a minute without a significant drop in pressure. A single cappuccino or latte only needs to steam milk for 30 seconds or so. If you're entertaining guests, this is a must have. At this level, there is no downtown in the machine. The limiting factor is the barista (so you :)).
Above the $2000 range, you enter double boiler territory. The espresso machine has a two independent boilers: a dedicated boiler to brew espresso and a dedicated boiler to generate steam. Now you can brew espresso and steam milk essentially forever. All commercial machines are usually dual boiler machines. Finally at the $3000+ range, instead of manually having to refill the water tank, the espresso machine can directly plug into the water pipeline like your sink.
Now, after ALL that, something like the Rancilio Epoca costs $5000. What do you get? You get all of the above, but as you pay more, you get more boilers so you can have multiple people work at the same time. Multiple brew heads, multiple steam wands, all independent of each other that can steam gallons of milk while pouring shots across all the brew heads at the same time. What does it feel like?
At the push of a button, hot water pours out of the grouphead at a precise and steady pace. The steam wand shoots steam at such high velocity that you can feel the air the steam is displacing. You can steam forever. Steaming a cappuccino with silky smooth latte art quality milk is almost automatic. Creating latte art requires steaming the milk so that foam is integrated into milk. This requires enough steam power to force the milk to violently tidal wave into the milk pitcher creating a blissful mix. Imagine dropping a spoon into a glass of water consistently. The Rancilio Epoca not only does this, but can force the milk to jump out of the pitcher if you're not careful. Beautiful.
Let's not forget our end goal though. How does the espresso actually taste? Does it produce any noticeable difference in quality? That, I'm not so sure. At the espresso machine side, what you're really looking for is an E61 grouphead, a boiler that can keep a steady 200F temperature, and a steam wand that can create latte art. Even at the $500 level, you already get those three features. You're really buying more fine tuning and a faster turnaround time between drinks. Past that point, the biggest differences come in from a) the freshness of the beans and b) your coffee grinder.
With my short little time playing with the machine, I can't tell if it produces a better drink. However, I can say, it sure is fun as hell to use.