So with only some further ado, there's been talk of Schilling's and Mike Mussina's Hall of Fame candidacy recently. (The comment I posted was in response to a discussion of the latter's.)
Judging Hall of Fame candidacy requires comparing pitchers from different eras, and ERA+ is a one-stop metric that shows how good a pitcher is relative to the league (and thus era) in which he pitched.

More precisely, and to quote Joy of Sox from the same post,
"ERA+ is the ratio of the league's ERA, adjusted to the pitcher's ballpark, to that of the pitcher. 100 is league average. Top-shelf starters will finish their seasons with ERA+s around 140."
For completeness, here's the
all-time career list. For relevance, perhaps gerrymandered, here's a condensed list. Pedro Martinez, at 157, is far and away the best all time. Roger Clemens, that cheating douche, is at 143. Johan Santana 141. Sandy Koufax, 131.
Tom Seaver, a Hall of Famer, is at 127, which is the same ERA+ as Curt Schilling and John Smoltz. Jim Palmer, a HoF'er, is at 126.

Juan Marichal, who's in the Hall, has a 123 career ERA+. Mike Mussina has a 122, which is better than HoF'ers Don Drysdale's 121 and Warren Spahn's 118.
In the comments of his post, Joy of Sox suggested that Mussina's comparables are Schilling and Marichal.
So, that's the backstory.
According to my calculations, and by 'calculations' I mean 'copying from columns I see',
Mussina's 6 best full seasons for ERA+ are 163 (in the strike year of '94), 157, 145, 142, 137, and 134 (and no more above 130.) But he's also had 6 seasons at 109 or lower, including 3 out of the last 4 under 100 (not including this current bounce-back year.)
Juan Marichal's 5 best seasons of ERA+ were 169, 168, 167, 144, and 132, (and no more over 130), and had only 4 seasons of lower than 113 (not counting his last two seasons, which totalled 60 innings), though 3 were under 100 (98, 97. 95)
As a starter, Curt Schilling has 4 seasons in the 150's, 2 in the 140's, and 3 more in the 130's. In seasons with at least 90 IP, he's had just 1 season under 100- 99 in '93- and just one other under 120.
It looks to me that Marichal's peak was better than Mussina's in the same amount of time, and Schilling has been better for a longer period of time than Mussina.

Burt Blyleven has just missed the cut for the Hall many times, and is the recent archetype of the borderline Hall candidate. His career ERA+ is 118, but a quick glance at Burt Blyleven's ERA+ numbers show him to be the better comparison to Mussina. Blyleven recorded 6 seasons over 130 (158, 151, 144, 142, 140, 134), but 7 under 110.
I don't know where exactly the cut off should be, but it might very well be around Blylevan and Mussina. And as of now, Blyleven has 1000 more K's than Mussina. Mussina might reach 3,000 k's (he's around 2700), and he might get pretty close to 300 wins, which obviously would help his candidacy.

At its best, the Hall might represent the dialectical synthesis of statistics and legend, of quantity and perception.
And I think the numbers show Mussina to fit a certain perceptual type; not quite the ace, never the very best pitcher, not the The Man you want on the mound in the big game, but instead a very good pitcher for a long time.
Schilling, on the other hand, has the regular season numbers to match his well-documented postseason heroics; for an entire decade, he was one of the very top pitchers in the game, and when healthy, never mediocre. I think Schilling is in.

One way to look at the debate is whether certain milestone numbers are both necessary and sufficient for Hall entry, or whether such numbers are only necessary, but insufficient without the stuff that legends are made of.
In other words, there's a symbolic debate here; is the Hall exclusively for the very best, those few players of each generation who attained the revered status as The Man, the kind of guy the old timers tell tall tales about, or is it also for those players who performed well-above league averages for a long period of time, but never reached the heroic levels that encourage the formation of legends in addition to statistical analysis?
I don't know. I just work here.