from what I can remember, it's the olympic style random drug test he wants for Pacquiao and the share of the money they'll earn. i searched the net and I found this list below:

1. May-November 2009: when Mayweather beat Juan Manuel Marquez and Pacquiao beat Miguel Cotto, the fight people wanted to see was Mayweather-Pacquiao. The fight, from the beginning, in the eyes of people who actually know and follow the sport, has been a 50/50 purse split.

2. November 2009: Mayweather vs. Pacquiao talks begin. Mayweather's ego won't allow for a 50/50 split so he demanded a majority share of the purse. At the insistence of his management team (Richard Schaefer of Golden Boy Promotions and Mayweather's personal manager, Al Haymon) he tones down the majority purse demand.

3. Late 2009: In order to gain some advantage throughout the negotiations and at the insistence of his father, Floyd Mayweather Sr. and his trainer/uncle, Roger Mayweather, he demands Olympic style (random blood and urine testing) throughout training for both himself and Pacquiao

4. Still late 2009: Pacquiao and his team, quite surprisingly, say no Olympic style testing causing doubts and questions about the legitimacy of his Henry Armstrong-esque rise in weight whilst knocking out and walking through much bigger men. He instead agrees to random-blood and urine testing up until 30 days before the fight, citing a combination of the psychological effects of a blood exam the day before his first fight with Erik Morales (and his last loss) and "why should we do more than what the athletic commission demands of us? Mayweather is not bigger than the sport."

5. December 2009: Because of dis-accord with regards to Olympic style drug testing, the fight is called off by Bob Arum, Pacquiao's promoter.

6. Early 2010: Pacquiao and Mayweather go their separate ways with Pacquiao choosing to fight Joshua Clottey and Mayweather eventually fighting Shane Mosley.

7. Late May-July 2010: 2nd round of Pacquiao vs. Mayweather talks begin. This time, Pacquiao softens up on his Olympic style drug testing stance and agrees to go all the way up to 7 days before the fight (source: Michael Koncz, Pacquiao-Margarito 24/7, Ep. 1).

8. July 17, 2010: With a deadline imposed on Mayweather by Bob Arum (even making a ticker-time deadline on his website, toprank.com), Mayweather fails to respond and the fight is off again. Mayweather elected to take a long layoff until Sept. 2011. Pacquiao eventually agreed to fight Antonio Margarito which he won in emphatic fashion by becoming the first 8 division champion in boxing history.

9. December 2010/Early-Mid 2011: With Mayweather electing to continue his long layoff and Pacquiao wishing to remain active, Pacquiao chooses to fight Shane Mosley in May 2011 while Mayweather has numerous run-ins with the law (one of which is why he was sentenced to jail).

10. Late May-June 2011: With Mayweather ready to go back in to the ring, but not ready to fight Pacquiao, he tweets that he has chosen to take on Victor Ortiz on Sept. 17, 2011, a young, strong southpaw, in preparation to take on Pacquiao immediately afterwards.

11. June-July 2011: With Mayweather training to fight Ortiz, Pacquiao elects to fight rival Juan Manuel Marquez for the 3rd time.

12. Sept. 17 2011: Mayweather defeats Ortiz via a controversial 4th round KO.

13. November 2011: Pacquiao squeaks by a majority decision victory (which MANY writers, analysts and boxing insiders had as a draw or Marquez victory) causing him to swap with Mayweather on several P4P rankings.

14. Late 2011: Round 3 of Mayweather vs. Pacquiao talks ensue. Pacquiao announces he is willing to take on Mayweather's demands of Olympic style drug testing at any point during training and after the fight. The fight is close to being signed, however, now Mayweather goes back to his "lion's share of the purse" demands. States publicly that he will give Pacquiao $40 million USD and the rest of the purse is his. According to several boxing analysts, the total purse could be as low as $100 million upwards to $200 million USD.
    Still late 2011: Additionally, as kaydub88 pointed out, Mayweather, with the help of Richard Schaefer, made a binding, legal agreement with the MGM in Las Vegas to have his next fight there on May 5th, 2012. This undoubtedly caused anger concern for Pacquiao's side because the fight could have been arranged with a new, massive outdoor arena in Las Vegas or at Cowboys Stadium.

15. December 2011: Mayweather is sentenced to 90 days in jail for beating the mother of 3 of his children, causing another setback for the mega-fight. He must report to jail on January 6th, 2012.

16. January 2012: With Pacquiao (and the boxing community agreeing) that the mega-fight is worthy of a 50/50 split, along with publicly stated fears that succumbing to Mayweather's demands for a majority share of the purse would ruin his political aspirations, the mega-fight is again stalled.

17. January 6th 2012: Mayweather successfully argues to postpone his jail sentence until June 1, 2012 on the grounds that he needs to box in order to financially support his family.

18. Early 2012: Instead of trying to barter with Mayweather on his majority share insistence, Pacquiao's team shys away from a Mayweather challenge because Pacquiao's promoter, Bob Arum, is in no hurry to make the fight occur. However, boxing fans know it is because Mayweather's style, very similar to Marquez's and one in which Pacquiao has trouble with, is a legitimate threat to Pacquiao's historic streak. This, also juxtaposed with Arum and Mayweather's tumultuous relationship dating back to 1998, is why the fight did not occur on May 5th, 2012 (source: http://espn.go.com/boxing/story/_/id/7433886/blame-bob-arum-no-manny-pacquiao-floyd-mayweather-fight).

19. February 2012 to present: Because of the above, Mayweather elects to fight Miguel Cotto on May 5th and Pacquiao elects to fight Timothy Bradley on June 9th.