Before we get into the complicated stuff, let's establish a baseline ...
What kind of pitcher was Pettitte? How does he stack up against other pitchers?
Thanks to the magical powers of Baseball-Reference.com's Play Index, I made a list of pitchers with between 220 and 260 wins - there are 27 of them since 1901 - and checked to see where Pettitte ranked among them in various categories.
With 240 wins, he's tied for 10th with Frank Tanana. Of the nine pitchers ahead of him, six are in the Hall of Fame; Jack Morris (254), spitballer Jack Quinn (247) and Dennis Martinez (245) are not.
With a .635 career winning percentage, he's third, behind only Whitey Ford and Three Finger Brown (both Hall of Famers). The three pitchers behind him are Hall of Famers, too. David Wells is the only pitcher (other than Pettitte) in the group with a .600 winning percentage (.604, actually) who's not in the Hall.
With a 116 ERA+ -- essentially ERA adjusted for league context and park effects - Pettitte ranks ninth. All eight pitchers ahead of him are in the Hall of Fame, but just behind him are Luis Tiant, Jim Bunning and Jack Quinn; only Bunning's in the Hall.
There's nobody exactly like Pettitte, so these comparisons can take us only so far. But I think it's fair to say that most of the pitchers who seem at least vaguely similar to Pettitte have been elected.
Granted, some of them had to wait an awful long time. Waite Hoyt - who won 237 games, 112 ERA+ -- last pitched in 1938, and was enshrined 31 years later.
On the other hand, Herb Pennock - who won 241 games, .598 winning percentage - was enshrined only 14 years after retiring in 1934.
I bring up those two because they had something in common with Pettitte: While both pitched for other teams, both gained most of their fame as New York Yankees during a period when the Yankees were the best baseball team on the planet. This helped them win games, and it helped make them famous.
Having pitched for the Yankees for so many years is going to help Pettitte, obviously. He won more games because he pitched for the Yankees, and he won more postseason games because he pitched for the Yankees. Does he deserve a bunch of bonus points for his postseason action? Not really. His postseason stats are almost identical to his regular-season stats. But some bonus points, yes.
Pettitte's fortunate that he kept his ERA below 4. Otherwise, it would be difficult to separate him from David Wells, who won 239 games with an outstanding winning percentage, and pitched for the Yankees (if not for nearly as long).
By the Hall of Fame's historical standards, Andy Pettitte is a marginal candidate who did everything well but nothing brilliantly, and would probably be enshrined after a moderately long wait.
But the historical standards can tell us only so much. The BBWAA's Hall of Fame ballot is going to be exceptionally crowded for many years, probably too crowded to allow for Pettitte's entry through that door (not to mention his status as a PED user). Historically, marginal candidates like Pettitte were elected (if they were) by the clubby and incestuous Veterans Committee. But the Veterans Committee - actually, it's Veterans Committees now - has gone through a great number of changes in just the last decade, and will probably go through more before Pettitte's eligible for consideration via that body. Without knowing what the process will look like in 20 years, it's almost impossible to guess what will happen to him.
Would I vote for him? That's a tough one. There are pitchers like Pettitte who aren't in the Hall, but there are probably more like Pettitte who are. I think I probably would vote for him, with those 19 postseason wins just pushing him over the bar.