First let me start out by painting this article with a very broad brush: If you're not a fan of B.J. Penn, you're not a fan of MMA. A non sequitur in a philosophy classroom, perhaps, but not in the real world. I know we all have our favorite fighters, but on the question "who speaks for MMA?", I can think of few better candidates than B.J. Penn.
This is not to say he was the best. Because he wasn't. But in boxing, the description for the sport has been synonymous with 'the sweet science'. MMA can't make such a claim. Why? Well look at our fighters. Some can box, but can't wrestle. Some can wrestle, but can't box. Others barely qualify as athletes. And certainly fewer can put the skills together in such a seamless way. The 'jack of all trades' principle that defines the identity of the mixed martial artist just doesn't allow for an approximation of what MMA might consider a "science".
However, Penn is that approximation. Before Penn ever entered the sport of mixed martial arts, he became the first non-Brazilian to win Mundials gold after beating Edson Diniz in the Finals (which you can watch here). This was only three years after entering his first grappling tournament. Grappling aficionados sometimes criticize Penn's Mundials win because he won the year Royler Gracie (who won the previous three years), and Fredson Paixao (who would take gold the following two years) were absent. But it's a hollow criticism. Given the amount of time Penn had been training, it was every bit as amazing as the UFC has advertised. Even so, for the purists, it's worth noting that the year before, Penn won bronze at a weight class above what he'd typically compete in, and his defeat came against an icon in Fernando 'Terere' Augusto (in a bout that is said to be competitive, in which Penn lost by an advantage, though I don't know much about the match other than rumors).
Of course, we've seen this story before: "grappler tries hand at mixed martial arts". Rani Yahya, Roger Gracie, Marcelo Garcia...the list goes on of grappling experts who become dartboards for 'real' fighters. Even those at the elite level, like Shinya Aoki, still haven't found a real rhythm in trying to apply their grappling prowess in an MMA context (as counter intuitive as that sounds). Not so for Penn.
In fact, this was sort of Penn's hype: His first three fights, fights fought in the UFC no less, were won by TKO or KO in a combined total of less than 8 minutes. His first loss, which occurred against Jens Pulver, was a learning experience. It was a close fight that Penn lost by choice (faded late in the fight), and by chance (he had secured an armbar to end one of the early rounds,and it's not fanboy hyperbole to say that not a second or two more, and Jens would have tapped).
When Penn got another crack at the UFC LW title, he would do so against Caol Uno, who found success against Penn in the clinch, scoring sporadic clinch takedowns, and minimizing Penn's offense on the feet (this was, of course, in very stark contrast to the blitz from Penn that put Uno in a folded corpse of a mess in their first bout). The draw that resulted essentially ruined the LW division (I should point out that the decision was terrible: not a memorable fight, but Penn clearly won 3 rounds).
BJ rebounded by wining two titles back to back: one at LW at Rumble on the Rock over Takanori Gomi and another at WW, against Matt Hughes at UFC 47. On the former, it's worth noting that Gomi was a big deal even then. He hadn't yet developed the striking game that would make him famous, but he won the Shooto WW Championship against Rumina Sato 2 years earlier when Sato was still damn good. On top of that, his striking was what should have given him the win over Joachim Hansen, who he had badly hurt at one point (which would have made Gomi undefeated).
It was a great win even at the time. But it's the win over Hughes everyone remembers. Penn was given no chance at 47. Hughes was a massive WW, and Penn was a small LW. Many fans on the internet mocked the fight, yes, rooting for Penn, but also the additional weight Penn was carrying around his waist to make 170, which clearly wasn't his natural weight. BJ's subsequent victory put Penn on the map as a force of nature. But it also started Penn's awkward walk on the tightrope between the 'prodigal' and the 'plebian'