Nick gets away with his style in mma for a very good reason, here's why: All of his opponents are much smaller than him, giving him a distinct and overwhelming reach advantage. Normally, the solution for a smaller fighter is to close the distance and stay on the inside of a larger opponents punches, that said, nick's opponents remain in his optimum striking range for fear of his bjj skills (which, even though it pains me to say it, are fucking top notch). If he fights a boxing match, that threat is gone, so I see any good boxer closing the distance and kicking his ass. Nick should understand that it's his bjj that scares his opponents into standing back in his range rather then the superiority of his boxing.
That's all well and good but it belays the real reason Nick Diaz's style works in MMA.
The vast majority of MMA striking is technically terrible.
Take someone like Scott Smith. He's a decent MMA striker... notably for KO wins over Noke, Martin and Radach. Yet his standup is almost entirely based around the fact that he hits hard. He wings punches, doesn't have a jab to speak of, has non-existent defence and the only variety in punch technique is because he's off balance from throwing the last one. Against someone throwing single shot haymakers someone like Diaz, who has the modicum of boxing knowledge needed to use distance effectively, throw to head and body with regularity and not rely on single Hail Mary punches, should and did take him apart.
The pattern repeats itself; Gomi (who was about even with Diaz standing for much of their bout) throws single wild shots, Lawler throws huge bombs, Frank "I'm a knockout artist now" Shamrock attempts to throw big punches, Zaromskis may have slightly more varied striking but has always been incredibly wild, Cyborg may have slightly toned himself down from his Manhoef days where he essentially locked his arms at a 45 degree angle and rotated his torso very fast until he or his opponent fell over but he's still a wild brawler.
The person who's given Diaz the most trouble standing also happens to be one of the few MMA fighters who also happens to have a little technique standing... and Diaz has 20 odd lbs and inches of reach on him. That's not a shock right there... Diaz's style is asking for trouble against someone who doesn't resort to the throwing of bombs from "crazy angles" and has learnt there's more to standup fighting than having "bricks for hands".
Add in that there are very few MMA fighters who can effectively defend themselves. There's virtually no counter-striking which means that once Diaz starts putting together one of his 9 punch combos he's free to finish it, little head movement and perhaps the cardinal sin... when hit most MMA fighters who aren't shooting for the takedown go straight back until they hit the cage at which stage they put their arms around their head and wait for the barrage to stop. It's then that Diaz gets to do his best work, boxing their ears, throwing the odd uppercut but mainly brutalising the body.
More power to Diaz. He's found a style that maximises his talents (reach, work rate, ability to not really have to worry about getting put on his back) while minimising he weaknesses (limited power, poor head movement) and its been (and will probably continue to be) highly effective. Daley will be a test; he's got huge power compared to Diaz's other opponents and at least some technique (even if he still rarely throws anything other than a hook or uppercut) but even then he's still basically a Hail Mary puncher and Diaz has a huge reach advantage.