Where in the Quran does it say anything about interest?
There is a Sura, in which states that there are people who say, Interest is like buying and selling.
and allah replies, but Allah has allowed buying and selling and forbidden raba (interest)
I do not know which Sura, but I heard it a lot.
and to know how big this sin is you must refer to the Hadith.
"Those who use interest, on the day of judgement, the prophet will say, here take this weapon and go fight against your lord. For you have waged war on Allah and his prophet when you made this act.
Why is it so haram? Banks do very bad things with interest money ;) ;)