Mussina had the misfortune of pitching during a time in which we saw a near-unprecedented run of pitching talent with the likes of Maddux, Glavine, Johnson, Pedro, etc. I don't think that necessarily should keep him out of the Hall.
For me, a HOF pitcher can come in a variety of forms. You have the all-time greats like Maddux and Johnson that combine fantastic peaks with long careers. You have the guys that lack a long career but had fantastic peaks, like Pedro. You have the pitchers with great career value but no peak, like Nolan Ryan. Then you have the pitchers that were consistently well above-average but never at the very top of their league, like Mussina.
Mussina gets in for me on the back of great career value. He's clearly far below the level of Maddux and the like, but he's above the established standard of pitchers in the Hall.
Jay Jaffe of Baseball Prospectus developed the "JAWS" system to measure HOF candidacy, based on Wins Above Replacement. It takes a player's career WARP score and mixes it with his peak WARP score (best 5 or 7 seasons, I forget) to get his JAWS score. He then compares it to that of the average HOF pitcher.
Here's a look at a bunch of the modern pitchers:
Code:
Pitcher Career Peak JAWS
Roger Clemens 199.6 83.9 141.8
Greg Maddux 180.3 86.0 133.2
Randy Johnson 147.0 77.3 112.2
Tom Glavine 137.4 63.7 100.6
Pedro Martinez 118.0 68.8 93.4
Mike Mussina 117.8 64.3 91.1
John Smoltz 122.8 58.5 90.7
Curt Schilling 110.3 65.9 88.1
Avg HoF SP 106.0 67.2 86.6
Mussina is below the peak threshhold, but solidly above the career threshhold, which jives with what I said about him being consistently well above average for a long time, but never incredibly great.
I'd personally vote for each of those pitchers (not solely because of this), and I don't consider myself a "large hall" type of guy. But, considering the size of the league now with 30 teams, I don't think 8 Hall of Fame pitchers from one generation is all that out of whack.