There are far too many vague, yet commonly used terms in the beer world. Most people would struggle to explain what smooth, light, heavy, or drinkability actually mean, but there's one term that is the most questionable and pervasive: Balance. Often the expressed goal of brewers is to create balance, even though it has the most fleeting definition on the list. Ask any beer lover to describe a current favorite brew and balance will probably be mentioned, whether it's a double IPA, gueuze, oatmeal stout or hefeweizen. The problem is that it's unclear exactly what entails that balance. The most common conception of balance is malt sweetness versus hop bitterness. Balance, in anything, is rarely that simple though, and we're talking about the millennia-old beverage that may be the single commodity most responsible for the beginning of written record keeping and civilization. So clearly, balance has been reached in beer in many different ways long before the current popular styles existed, especially since hops have only been used in brewing for one-thousand years.
There are plenty of great beers that can only be described as unbalanced according to the bitter and sweet dichotomy. Many prized IPAs are dry and bitter with a subtle malt flavor that is not even noteworthy. Stillwater's Classique farmhouse ale, for example, is similarly unbalanced by malt; bland corn and rice are used in this beer, clearing the stage for flavorful saison yeast and American hops. Sour flavors can fill in for the bitter ones, too. Beer can have acidity and peppery citrus like a good salad dressing, then a crisp crouton malt bite to "balance." It also could be fermented with fruit for further complexity, like building a salad. The elements of balance in beer like this are completely different than those of an English pale ale. A golden fried food with a squirt of citrus is a much more apt metaphor for that style. I think comparing beer to specific food dishes is helpful, if imperfect, in understanding different ways a beer can achieve balance because the 5 basic tastes can exist in beer, not just sweetness and bitterness. A dry-hopped gose, (a tart, salty ale) might even exhibit sweet, sour, salty, bitter and umami flavors all at once, like the complex Pad Thai dish. Harmony might be a better term than balance because it leaves possibilities open that are beyond those two tastes most commonly connoted by that term.
In the past, brewers had to balance out their brews using whatever resources were available to them, and also in accordance with local tastes. This process has changed with globalization and modernity, but I don't know if it's changed much. Regional tastes are still very distinct, like West Coast versus East Coast IPA, or Czech versus German pilsner, and these variations are still influenced by local demand. When brewers construct a beer that resonates with local consumers, it is usually because a unique, local ethos is perceived in the beer itself. This ethos may be felt by drinkers because of local ingredients, a unique brewing method, or just an attitude presented by the brewery. The birth of the modern American IPA in San Diego most likely came about because brewers and drinkers in southern California felt the bold innovation of brewing beers more bitter than ever before represented their community mindset in some way. They threw traditional ideas of balanced bitterness to the wind and, to traditionalist brewers' surprise, the in-your-face style now rules the American craft beer scene.
Obviously the tastes and available ingredients, not to mention brewing technology, have varied widely throughout the world and different cultures have found different solutions to the issue of balance. In the not-too-distant past, the matter of old beer turning sour was dealt with by adding fresh, sweet beer. The balance reached through this technique led to one of the first phenomenons of beer in the western world, the porter. It was the most popular beer in Britain and it's colonies for at least a century, and strong, or stout, versions of porter were exported to Russia and Baltic countries, spawning even more beer styles. Blended beer isn't made in large quantities anymore, except in Belgium where millennia-old methods of brewing have survived. This is brewing like it was done before yeast was fully understood as a species. The conquest of yeast is perhaps what led to the current cornering of balance.
Now most large craft brewers use a single ale yeast for most of their beers, a yeast whose behavior and flavor they know well enough to have complete control of it. This creates an almost blank slate of fermentation upon which to place the malt and hops. Unless it's a yeast-forward style like most Belgian ales, the German Hefeweizen, or bacterially soured like the Berliner weisse or barrel-aged styles, most beer balance is looked at as a sweet versus bitter affair. The cloistering effect of technology seems to be how we got to this pervasive conception of balance. There are many ways to look further into beer than this narrow "balance" beam, the first is being aware of all the elements that contribute to the drinking experience of a specific beer. Salinity, pH, and hardness of water have a great effect on flavor and mouthfeel, too. "Burtonised" water has gypsum added to recreate the beer of Burton-upon-Trent in England which had naturally hard water that was perfect for accentuating the crispness of their pale ale, a character not attainable by thinking primarily about hops and malt. So, that refreshing finish in your beer may have to do with elements you don't even think to consider. There are endless subtleties leading the harmony in your glass, even if the dominating pillars of the experience are hops and malt. These other elements often set great beer apart from good beer. The chosen technique of brewing itself can create different kinds of balance, too, almost regardless of the ingredients. Beer is a lot more than just a hop tea sweetened with malt syrup, and it deserves to be drunk with that in mind.