1. They're fucking obnoxious style of play. It's called Catenaccio. The whole entire methodology for Italy for playing in these tournaments is to score the first goal (many times by diving for a penalty), and then pass the ball around on the backend, and almost never attack - thus wasting about an hour or so of your life. They pit about 8 guys in the box at all times and just fuck around. They will almost never attack or try and score a 2nd goal in a match unless the other team just can't do anything, or they go on a clear counter attack. Greece uses almost the exact same tactics and that's why they're boring as fuck as well.
http://en.wikipedia.org/wiki/Catenaccio
It's quite possibly the most uncreative tactic in the game, and it's what makes so many people hate the sport.
2. Watch any game involving anything Italian, and you will see blatant cheating and diving. I would rather watch 10 hours of Barney the Dinosaur's "I love you - you love me" songs being sang than sit through 10 hours of Italian football. Seriously, in Guantanamo, instead of playing 24 hours of hardcore death metal, they should torture inmates with 24 hours of italian football matches from over the years. They'd probably fess up to anything at that point.
Watch an match - particularly any important one - that involves Italy, or an Italian club team, or an Italian player. A good example was the Rome Derby (probably the last serie A that I watched about a year ago). I turned on espn3, i watched for about 20 minutes. I saw about 24 seconds of actual play. 1 Goal kick, 1 header, about 3 passes and then a dive... then about a minute and a half of agonizing bitching and moaning and sitting on the ground like a 4 year old. I had to turn it off. It was fucking RIDICULOUS.
3. ...and then the matchfixing, and money scandals. These go back nearly to the inception of the game in Italy. Players, owners, referees... all paying off refs or players to do stupid shit during matches that just screws up the entire game. You might as well watch Cspan or something, because Italian soccer is about as rigged and corrupt as anything you'll ever see. Teams have been relegated to the 2nd division, others have been banned from the game.. Imagine if Pete Rose and Bounty-gate happened every other year on a MUCH larger scale.. That's Italian football for you.
4. Oh, and lets not disregard the fact that they all look like pretty boy bitches to begin with... I swear some of them must wear makeup to the game... It's like watching the cast of Jersey Shore play football. Seriously...
And that, is why many people who actually like this sport, fucking hate Italy.