If I were to throw a 30-40 pound iron door at someone, what dice would I roll, and what modifiers would apply? Would someone be at disadvantage? Let's say that I have 20 strength, 13 dexterity, and there's someone about 30 feet away. What's the DC?
Up to the DM
I assume you are trying to do that as an attack. Although the books have rules on Improvised Weapons that could apply here, due to the weight of the mentioned object (and the not-exactly-comfortable-to-hold-and-throw shape), the DM could require an Athletics Check just to see if you are able to do that, to then resolve the attack as an Improvised Weapon.
For thrown improvised weapons, we have
An improvised thrown weapon has a normal range of 20 feet and a long range of 60 feet.
So, by throwing it at 30 ft away, you do have disadvantage since you are throwing it farther than the normal range. Since it's an improvised weapon, you don't have proficiency with it, thus the Attack Roll modifier is just +5 (your Str modifier), and the damage is 1d4 + 5 (1d4 + STR). The DM could obviously rule the damage differently.
About the DC, it is again up to the DM. While certainly with a score of 20 in Str you can lift or carry the door, throwing it might prove harder. Still, 30-40 pounds is not that much, especially for some adventurer used to, you know, axing dragons. My personal guess would be a DC of 15 to 20, but again, that's completely up to the DM and YMMV from table to table.