Everyone is going to tell you the same story over and over again: Pacquiao won with youth, head movement, perfect range, and most importantly speed. It was absolutely stunning to see De La Hoya completely overwhelmed and helpless as Pacquiao cut angles and fired shots from different locations and in different rhthyms. For fans of MMA who don't watch boxing or didn't see this fight, think of Pulver vs. Faber. Faber was able to win by being a quicker, superior athlete whose offense was unpredictable, smooth, too fast and too much. Pulver was never really able to engage Faber with any sort of sustained offense as Faber moved, turned, deftly transitioned in and out of range and remained elusive all while firing right hand leads amidst a mix of unorthodox strikes. I hesitate to make comparisons particularly across sports, but the key idea is that movement, timing, speed and properly selected offensive opportunities carried the day.
Without question, Manny Pacquiao is the best boxer in the world pound-for-pound. I don't know that beating De La Hoya - given the impotent performance De La Hoya turned in - necessarily means that Pacquiao's victory over him was a win over the most highly ranked or uniquely challenging opposition he could've faced, but the idea is that he jumped enough weight classes against a high-profile opponent to test his skills in the untested and categorically daunting waters. What transpired was an absolute beating and further proof that his skills, on some meaningful level, translate across divisions.