Finance, basic accounting, economics, and strategy. A lot of it boils down to thinking about cost/benefits all the time. A lot of these concepts are common on this HN board because it is entrepreneurship themed. Examples:

"Is it okay to build a cheap thing today, knowing the engineering sucks and it will be expensive to fix later?"

"Technology X is cool, what is the cost to switch? Short term, long term? Support costs? People costs? Are there savings? Are there strategic advantages?"