Well, at least they admitted it was a mechanical. I've found that a lot of airlines these days avoid telling you anything about delays and cancellations so that they can avoid actually lying about it when it's a mechanical to keep from giving you a hotel voucher.

Did you get the names of the people you spoke with? That's always the best when writing a complaint letter, because that's what you'll have to do. You might get a travel voucher out of it. I doubt Delta will give anything, however.