Sweatpants in Public.

When is it appropriate, and when does it look like you're a slob?

Trick question; it's rarely "appropriate" and you hardly ever DON'T look like a slob. That's just the way it is.

Examples:

1) College campuses during the day, in classes.
Not appropriate, you look like a slob.
I must state that when girls wear this article of clothing, it's a whole different look. That's a fashion matter, so for purposes of discussion let's exclude them from this section.
It's college people. We know you have SOME money. I don't care what you look like, or who you are, put on some real pants. Invest in something that's not made of cotton. It's worth it in the long run. You look like you just rolled out of bed, and as far as I know, that "look" went out a little while ago.

2) College campuses at night, about 11pm-anything before first classes.
Appropriate, you still look like a slob.
At this time, you are not frowned upon if you're wearing sweatpants around campus. It's night time and you're a college kid. Unless it's a weekend, you're perfectly able to rock this look without being thought of as a complete hobo.
Ladies, it's always ok for you to wear it as long as you look good in it. That's the rule of thumb. If you make a guy look you up and down for a hot second and they don't go "meh" than it's ok.

3) the real world, any time of day.
Not appropriate, you look like a complete moron.
I include everyone in this category, including Plaxico Burress (was that really 3 months ago?)
Don't do it. Unless you're living on the streets, you shouldn't be wearing sweatpants in public. It screams "I give up, this is the best I'm gonna do," like you wouldn't believe.

Now please, take this advice and make wise decisions about sweatpants.
It's for you're own good.