I believe we have a misconception of what constitutes a second-tier fighter in MMA by the mainstream audience, particularly in comparison to boxing.
I don't have any actual statistical date to support what I'm about to say. I am basing what I say on observations I have made over several years as a fan of both boxing and mixed martial arts, and as a participant in martial arts / hand-to-hand training.
I believe the talent pool in MMA is far, far larger than it is in boxing. There are good MMA schools all over the country now. There are BJJ/MMA gyms in places that are hours away from the nearest boxing gym. Instead of having boxing 'smokers' like you used to in the old days, you now have local MMA shows, even in very small towns or communities with only a few thousand people. So there are more people training, more people fighting, and thus harder to judge a fighter by his record when he makes his UFC debut with 7 wins and 3 losses. How good is he really? What has been the caliber of his competition? Truth is we (and even the UFC match makers) can't be sure til he fights a 'known' commodity.
Furthermore, 'promoters' like those in boxing do not yet have the reigns in MMA. They can't pick and choose who they want to ascend to greatness yet, quite as easily as they do in boxing. Even in organizations like Pride and the UFC, where they market certain fighters heavily, those favored often do lose to upstarts or fair poorly and only eke out a decision over previously 'unknown' fighters. The most recent case in point is not Matt Serra, who has actually been training in MMA for over six years, it's Sokoudjou, an Olympic black belt in judo who has just defeated two of Pride's best light heavyweights in back-to-back fights, Nogueira and Arona, by KO's set up with one punch. Sokoudjou's record before his fight with Nog? 2-1.
Because there are no 'boxing promoters' per se yet involved in MMA, you have no one coddling fighters to 20-0 records before they start fighting contenders. MMA fighters are fighting everyone from complete unknowns and contenders from the beginning of their careers. A boxer with a couple of losses on his record at say, 20-2, is almost a journeyman, whereas that's a great record for an MMA fighter.
The fact is there are far more ways to lose in MMA than there is in boxing. It is much easier to KO someone with MMA gloves than boxing gloves. You can be KO'd by punch, knee, or kick, you can be cut by elbows sufficiently to order a doctor's stoppage; and you can be submitted hundreds of different ways by an opponent using any of your four limbs and your head and neck. It isn't surprising that an MMA champion will lose more often than a boxing champion when there are hundreds of more ways one can lose, they're fighting against men drawn from a much larger talent pool than boxing, and the men they're fighting may be new to that organization but that doesn't mean they're new to MMA.
Many of the men who fight in MMA have well-established backgrounds in collegiate wrestling, kickboxing, judo, competitive jiu-jitsu, and other combat sports. Just because a guy is fighting in the octagon for the first time doesn't necessarily mean he has no combat sport experience. Fedor was a sambo player first, Cro Cop a kickboxer, Couture a wrestler, and so on. So a champion who loses to someone who the public thinks is obscure or an also-ran may say more about the public than about the fighter himself. You don't get cauliflower ears from training for 6 months and then stepping into the octagon!
There is also the problem of appearances. People are notorious for assuming that the fighter with the bodybuilder-type, more cut physique will dominate his opponent regardless of whether they know anything about the true skill levels of either man. The more people watch MMA the more they will learn that appearances can definitely be deceiving.
I have been following MMA a long time, but I still underestimate fighters and my predictions are often wrong. We watch a fighter get demolished in a fight on television. Two years later we see that same fighter again for the first time since then, facing another fighter we've been watching regularly for two years. We assume the one we've been watching will demolish the first fighter; after all, didn't we watch him lose to Joe Shmoe two years ago? But we haven't seen the fights he's had since then; we don't know what training he has undertaken to fix holes in his game; we don't know what kind of shape he is in, his current mental state, none of that. It's the nature of the sport right now because there are so many fighters, so many organizations fighting, that we cannot know and keep track of who everyone is fighting and the caliber of their competition. But boxing has so few good fights by comparison it is easier to judge who we think will win. But upsets still happen - Maskaev KO's Rahman; Wladimir out points Sam Peter, and Sam Peter outpoints James Toney?
People point out that there are a lot of upsets in MMA; the truth is there are a lot of upsets in boxing, it's just that there are fewer fights on television than compared to MMA, and fewer fights still that even matter to the average fan. These days it takes a real fan of boxing to order a PPV; but people are regularly ordering an MMA PPV every single month. Six of the largest 8 PPV last year were MMA - not boxing.
So although some will say that GSP's loss to Serra, and other fights like it, is bad for MMA, I think otherwise. I hope instead that people will be inspired to learn more about the sport of MMA itself and the men who fight in it.
The only caveat that I will add is that the more popular MMA becomes, the more the mainstream takes an interest, the more we will see boxing-style influences enter the sport, and along with it, the caste system. So I say enjoy it while it lasts.