A few things to make fun of for every team in the league, should a nasty situation arise with a fan of somebody else and you don't want to think for yourself. Because sometimes being a total dick is the only satisfaction a fan of a crappy team gets to have.

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For Novices: Jeffrey Maier, Peter Angelos' iron fist, Beattie/Flanagan GM duo of terror
For Veterans: Brandon Fahey's manhood, fan demographic at Red Sox/Yankees games, irony of Frank Wren taking over the Orioles after Gillick took off
Avoid Mentioning: Mike Hargrove

For Novices: Inability to win the division, Yankee superiority, ridiculous accents
For Veterans: Manny Ramirez's defense, parasitic nature of fan base, godawful ballpark, pink hats, Joel Pineiro
Avoid Mentioning: Recent success, Heathcliff Slocumb, cancer

For Novices: Bobby Jenks devouring Mark Buehrle's talent, legally retarded manager
For Veterans: Tom Gamboa, Hawk Harrelson, top HR-hitting team in baseball preaching the importance of productive outs, Robin Ventura got his ass beat by Nolan Ryan
Avoid Mentioning: Recent success, Matt Thornton

For Novices: Jhonny Peralta's h, 1999 ALCS, 2005 pseudo-choke
For Veterans: Jose Mesa (be careful), curse of Rocky Colavito, Ten Cent Beer Night, whatever happened to Brandon Phillips, Drew Carey
Avoid Mentioning: Mike Hargrove, front offices, 14-2

For Novices: World Series outcome, Ivan Rodriguez's briefly-rumored homosexuality, Neifi Perez, 2003
For Veterans: Guitar Hero, Randy Smith trades for Juan Gonzalez, Dean Palmer contract
Avoid Mentioning: Recent success, Carlos Guillen

For Novices: Recent success, mascot that scares the crap out of little kids
For Veterans: Gil Meche, Appier/Damon/Dye deals, David Glass' eyebrows
Avoid Mentioning: Nothing

For Novices: Rally Monkey, anti-intellectual bandwagon fan base, ballpark environment best characterized as vaguely pedophilic
For Veterans: Laughable Darin Erstad extension, worship of productive outs, Jered Weaver's weight/appearance/talent, Mike Scioscia acting like a little bitch, 1995
Avoid Mentioning: Recent success, big words

For Novices: Kirby Puckett doesn't belong in the Hall of Fame, inability to win in the playoffs despite Santana
For Veterans: Torii Hunter's overratedness, Felix stayed healthy while Liriano didn't, Carl Pohlad is a giant douche, Puckett's personal life
Avoid Mentioning: Recent success, marginal wins

For Novices: World Series drought, Alex Rodriguez in the playoffs, Derek Jeter's defense, payroll, Chien-Ming Wang's name
For Veterans: Jaret Wright, David Ortiz and/or Dave Roberts, Ronan Tynan's ears, Jeff Karstens' jaw, parasitic fan base, Jay Buhner trade, The Double
Avoid Mentioning: Recent success, historical success, Arthur Rhodes

For Novices: godawful stadium in a godawful neighborhood, cheap owners, Eric Chavez's alleged "stardom", inability to win in the playoffs, Joe Blanton's circumference
For Veterans: Nick Swisher's porn staritude, crappy attendance, past incidents of bleacher racism, jackbag drummers in the outfield who need to shut up for one God damn second because Jesus quit it it's impossible to fucking concentrate
Avoid Mentioning: Recent success, trades, head-to-head season series results

For Novices: never won 71 games, only team to never see the postseason, Dennis Quaid
For Veterans: Rolando Arrojo holds three all-time franchise records, Rob Szasz's fifteen minutes, Doug Waechter, the McGriff/Canseco/Sorrento experience
Avoid Mentioning: Future success, 2-6-2 triple play last September

For Novices: Alex Rodriguez contract, Little League ballpark, Hank Blalock's stagnation, reprehensible "The Natural"/Fox Sports PA home run music, Tony Romo in Qwest Field
For Veterans: Nolan Ryan is overrated, Michael Young is overrated, Travis Hafner trade, similarity of Josh Lewin's psychological profile to that of a twelve year old girl  
Avoid Mentioning: Recent success, replacement of inadequate manager and GM

N/A; there are no Blue Jays fans, only Blue Jays players

For Novices: The National League sucks
For Veterans: The National League sucks
Avoid Mentioning: Recent success

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So there you have it, a brief guide for how to act like a belligerent asshole should a fan of another team lure you into a heated discussion. Me, I'm just not going to tell anyone who I root for, but to each his own. Just never tell me you weren't prepared.