1 - He played for some horrible teams, so there were a great many years that he was outstanding for teams that were way out of contention for much of each season.
2 - His postseason record isn't exactly sparkling. He generally pitched well enough to win, as his E.R.A. and K/9 would attest to in the postseason, but that record is 7-8.....fair or not, it doesn't exactly scream John Smoltz or Andy Pettite.
I think if the Yankees had either not blown Game 4 or 5 against Boston in 2004, they'd have gone on to beat St. Louis, and that W.S. ring would have elevated Mussina to no-doubt status. Without one, he is just lacking that 300-win or 3,000 strikeout milestone, which to a lot of people matters.
Personally, I think a guy in the Top 25 all time of Pitcher W.A.R. who finished in the Top 6 of the Cy Young 9 freaking times is an easy choice.