Guitars enjoy mass appreciation that far exceeds that of the violin. Millions play them and they are the foundational instrument for most popular music. Eric Clapton ( a guitar player) is far better known than Pincas Zuckerman (a violin player). Stradivarius is of course a well known name in violin making, but then so are Martin, Gibson, Fender, and even Guild equally well known as guitar makers. Therein lies a key to understanding the current situation that both liberates and constrains the acoustic guitar. An individual maker (or two or three) carries the standard for excellence in violins, while a number of huge factories provide the definition of what constitutes a good guitar. Individual luthiers who must make a living selling what they make are affected by the reality this creates. This may explain why many handmade guitars sound like good factory made guitars, and vice versa. And now, the understanding of line production developed by American manufacturers has been united with Asian labor to produce very OK guitars at even lower price points. This has pushed the intrinsic musicality of the instrument even further into the public consciousness because reasonably good ones are now more affordable than ever.
The vast majority of acoustic guitars are X-braced with a sound that is fairly characteristic of this approach to top design. The question of who invented X-bracing does not have a universally accepted answer. But it is apparent that sometime in the 19th century it began to appear in gut stringed instruments. There is general agreement that the Martin company was most responsible for adapting the design to steel stringed guitars in the early 20th century. The guitars they made during the transition period attract the most interest. The reasons are probably beyond anyone's ability to attain certainty, but we can speculate that the way they were built plays a role. Martin has long been famous for its warranty and most importantly, the fact it actually honors the contract with its customers. The earliest steel stringed instruments are more lightly built than later ones, but as the company gained experience with warranty issues, it began to build later versions heavier. When you build guitars by the thousands and guarantee them all for life you must allow for the probability that a number of them are going to fall into somewhat unreasonable use - or worse. Unreasonable, anyway, from the point of view of a fragile instrument built to take the fullest possible advantage of the intrinsic potential of the design and materials.
In any case, Martin did not have a lot of experience in making steel string acoustics at the time of their pre-war "golden era". The first dreadnoughts (nine of them) were manufactured in 1931. Smaller steel string instruments appeared as early as the 1923 catalog, but were not made in quantity until 1928. When current CEO Chris Martin discussed the decision to re-issue several of their pre-war models at affordable** prices - their sound being what everybody wants - he admitted one of the obstacles in determining specifications was the inconsistent nature of the pre-war production. That's not surprising for a company that was in the process of learning how to make the product work within the mandates of mass production and marketing. The transition from gut strings to steel strings resulted in lightly braced instruments that did not fit well with the company's lifetime guarantee and would require more attention to the emerging nature of each specific instrument as an individual specimen, as opposed to an instance of a "model". However, most agree their sound was more open than that of the steel string instruments Martin would eventually produce after it adapted to the mandates faced by any factory operation. Martins may not compare well with the responsiveness Ervin Somogyi gets, when heard by certain ears and expectations, but the majority of the guitar loving crowd regards their sound as the holy grail of acoustic, steel stringed guitars.
Because of the popularity of the Martin sound, especially that of early Martins, it is not surprising than many hand builders produce instruments that sound specifically like those early X-braced instruments. Martin has challenged everyone to attempt to copy them and has not patented the X-brace design. This has turned out to be a very smart marketing policy. The Martin sound dominates the market, except don't say that to a Gibson player (their shorter scale length does make a difference).
The major competitor to the X-brace system is ladder bracing, which is considered in the first blog on this list. That blog also deals, in a general way, with some of the differences between the two approaches.