I decided to run some numbers to figure out the impact that each team's offense, pitching, and defense is on pace to have over the course of a full season. Results are in the table below. This may not be the absolute best way to do it, but it was the easiest, and this is my lunch break. Let me know if I did anything wrong.
OFFENSE: |R/G - lgR/G| * 162
PITCHING: |FIP - lgFIP| * 162 (assumes 9 inning games, which is ~average)
DEFENSE: |THT +/- rating * 0.8| * 162/GP (GP = games played; +/- rating and GP from May 14th, as THT hasn't updated its Team page for some reason)
Note that these are absolute values. So, for example, San Diego's offense is on pace to be 207 runs away from the league average, while its pitching is on pace to be 50 runs away, and its defense 32 runs away.
The key bit of information is that last line in the table. Offense is set to be the most variable of skills across the league, followed by pitching, followed by defense. I don't think anyone will find that particularly surprising. However, while the impact of defense may be the smallest, it's still significant. The average team's defense is on pace to end up 34 runs away from the league average (that's confusing), which is somewhere between 3-4 wins in the standings. Toronto is on pace to be 14-15 wins better than Pittsburgh on defense alone. 14-15 wins! To say nothing of how much better they were than Tampa Bay and Florida a year ago in the field.
Defense. Defense defense defense. I talk about it all the time to compensate for the fact that so few other people seem to pay it any mind. Just because it's not the most important thing in the world doesn't mean it's not important, and it's so ridiculously easy to upgrade that there's zero excuse for not doing so if you're given the opportunity. It'll make a difference. It may not be as obvious to the naked eye as a home run or a strikeout, but it'll help far more than most people understand.
Let's pretend, for the sake of simplicity, that Jeremy Reed's offense = Jose Vidro's offense (Reed's better, but anyway). Putting Reed on the bench and leaving Raul in left field instead of putting Reed in left field and sliding Raul to DH would be roughly equivalent to leaving Chad Billingsley in AAA so you can keep Adam Eaton in the ML rotation. Do you think people would stand for that? Of course they wouldn't. And neither should we stand for this.