I haven't gotten to read it yet, but I have McDonald's "Giants of Chess Strategy" and Petrosian is one of the featured players that the book is structured around (the others being Capablanca, Nimzowitsch, Karpov, and Kramnik). Pretty good company to be among.
What is wrong with Petrosian?

There was a time when Petrosian was required (or maybe forced) to play many matches. He was exhausted, so he took an easy way out by taking draws from lesser masters. This is one of the reasons why Petrosian is unpopular now. However, Petrosian at his best is a formidable foe and a worthy world champion.

It's funny seeing unrated players finding Petrosian's style "boring" for the simple reason they cannot understand it.
Lock the pawn structure every time he had the chance, but he was a world champion. So, what can I say!

Petrosian was pretty much the farther of the positional exchange sacrifice.
according to chessgames.com here's his record in classical chess:
691 wins
1057 draws
156 loses
Petrosian was nick-named iron tigran. People also said it was easier to win the USSR championship than a game against 'iron tigran', the master of prophylaxis.

What I particularly love about the above Petrosian-Spassky game is that it demonstrates that Spassky as perhaps a bit lazy even in the 1960s when it came to match preparation. Take a look at this game played by Petrosian about a decade earlier. Spassky really should have known this tactic was in Petrosian's arsenal:

I don't have it here (my book collection is back in China, I'm in Canada), but there was a pretty cool game Tal-Petrosian from one of the Soviet Championships (1973 or 4, I think) that looked more like a bar-room brawl than a typical Petrosian quiet positional snooze-fest. Keres annotated it for Chess Life and Review, then it was included it in his collection of articles Power Chess. "Petrosian in Tal's Clothing", I think he called it.
Yup, I remember that article too!
His style is boring so his wins are unlikely to sell a lot of books. So you only see his losing games.
his style is far from boring, he was a very strong tactian, he would see threats before his opponents would even start to calculate.
his win against pachman is missing in my blog, i will add it the next days
https://improveinchess.wordpress.com
I looked at Tigran Vartanovich Petrosian vs Vasily Smyslov URS-ch 1961 · Queen's Indian Defense: Kasparov-Petrosian Variation. Petrosian Attack · 1-0
I have to admit that I dont understand why Smyslov had to resign. Is it the pin on the d6 bishop ? Obviously, Petrosian had an almost promoted pawn and 3 more free ones to come, but he was a bishop down. Wait, I think I just answered my own question, but comments are welcome.