Bottled beer is significantly different than the same label beer available on tap.
How does bottled beer differ from draught beer? What is done to the bottled beer in order to prolong its shelf life in the bottle?
And finally, why does draught beer taste so much better than bottled beer?
There was a study about this which concluded that tapped beer can actually be better for different reasons (although taste is rather subjective). The reasons are:
Question one: how does bottled beer differ from draught beer? This varies from beer to beer, and should be handled on a beer-to-beer basis. Sometimes beer in the bottle is pasteurized, while the keg is not. Sometimes one or the other is filtered, while the other is not. The gas content can also differ, since this is adjustable in draught systems but not with bottles.
In some cases, such as when yeast content is very important for the flavor profile of a beer, like German Hefeweizen, how much yeast you get in your glass has a big impact on the drinking experience. When drinking from the bottle, you can make sure that you get all of the yeast from the bottom of the bottle into your glass by agitating the last few ounces of beer, but this isn't possible with a keg. Kegged versions of these beers still have yeast in them, but you're not getting that perfect ratio in every glass. In my two years as a beer enthusiast living in Germany, these beers are almost always served from the bottle, and every bar or bartender has their preferred method for getting all the yeast out of a bottle.
Question two: What is done to bottled beer to prolong its shelf life? This also differs from beer to beer. Some imports are pasteurized in the bottle, some others, namely MillerCoors products, use proprietary hop derivatives like Tetrahop to keep hop flavor while minimizing hop oils' susceptibility to producing off-flavors due to light exposure. This isn't a bad or unnatural thing, and I hope this becomes more widespread, because skunked, lightstruck beer is awful. (see this interview) This, however, is also present in kegged beer, as it has other beneficial properties like increased head retention. Sometimes, though, nothing is done beyond packaging, like Sierra Nevada's transition from twist-off to pry-off caps a few years ago to combat leaking, infection, and oxidation. Brookston Beer Blog 2007
Question three: Why is draught so much better? Very, very subjective, and you may want to clarify or qualify this in the future. Is good draught beer better than old, skunked bottled beer? Yes. Is good bottled beer better than flat draught beer poured through dirty lines? Also yes. Each has its pros and cons when it comes to service and storage.
I would venture to say that much of draught beer's appeal comes from it being special- you typically have to go somewhere else to get it in a way that's not usually available to the home user, like how the theater experience differs from the home movie experience. Yes, you can get bottled beer in bars, too, but it's still a more "normal" format. In terms of taste over experience, though, I don't know that you could say for sure without some sort of blind tasting with lots of variables (age, glass type, line cleanliness, CO2 pressure) controlled for. Even then, what does it for the tasters might not do it for you.
For further reading on all that goes into draught beer, check out the Draught Beer Quality Manual
It's been a long time since I originally asked this question. While my contributing answer here doesn't really clear up why draught beer (maybe, maybe doesn't) taste better than bottled beer, two things are clear to me now after spending a lot of time reflecting on the difference:
One
As Tom pointed out in the very first comment, drinking beer (and coffee too, for that matter!) out of a small spout (bottle, travel mug, etc.) blocks your nose from inhaling the aromas of the beer. This limits the amount of flavour you can sense. Pouring the bottled beer into a glass dramatically improves its flavour because the open mouth of the glass allows you to taste and smell.
Two
There's an inverse relationship between perceived "better" taste and the volume of bottles a brewery produces (and how far they intend to distribute).
Beer brewed at a local microbrew has a shelf-life of only a few weeks on the outside; preservatives are limited or omitted and their ingredients are typically fresh, with a comparably short self-life.
Larger breweries distribute further distances, demand a longer shelf-life, and usually have a need to maintain a consistent taste across their entire production line and the resulting shelf-life. These breweries use high-volume equipment (an increase in the amount the beer is processed) and high-volume ingredients (preservatives in ingredients make them last longer, but may not be fresh). It also becomes increasingly difficult to quality-control when mass-producing — these breweries are less likely to take risks, and they rely on machinery to achieve consistency, often sacrificing quality.
tl;dr: draught beer, like micro-brewed beer was never intended to last for months or years on the shelf, and thus preservatives don't need to be factored into the production, and the beer is liable to be fresher more flavourful.
One last bonus point
I've also noticed that at times, bottled microbrews, poured into a glass can actually taste better than their draught counterparts. Go figure ¯\_(ツ)_/¯
