What did Martin Van Buren stand for?

Martin Van Buren stood in the tradition of limited Federal government as espoused by his predecessor Andrew Jackson. Like Jackson, Van Buren was a staunch advocate of states' rights, and he sought to interfere with what he saw as their fundamental prerogatives as little as possible.
Whatever the merits of such an idea on the theoretical level, in practice it proved disastrous for the country during Van Buren's single term of office. During the Panic of 1837, the irresponsible speculative lending habits of the state banks led to a serious economic crisis. This was largely the consequence of Jackson's banking policy—which Van Buren endorsed and continued—which removed funds from the Federal Bank and gave them to state banks.
Jackson was extremely hostile towards the Second Bank of the United States, whose charter he refused to renew. He and his supporters believed that the Second Bank—or indeed, any Federal bank—was an instrument of economic oppression in the hands of the East coast banking elite, and which would be used against the kind of ordinary people—farmers, traders, and small businessmen—who'd helped Jackson get elected. Instead, Jackson diverted government funds from the Second Bank to individual state banks in the belief that they would be more responsive to local economic needs.
It didn't quite work out like that, however. As we've already seen, reckless speculation led to economic chaos, for which Van Buren found himself hopelessly unprepared. Rigidly sticking to his overriding belief in limited government and states' rights, Van Buren refused to restore confidence in the banking system by reestablishing the Federal Bank. Instead, he diverted some of the government's funds from the failing state banks to an independent treasury.
This did little to halt the growing crisis, and the economy still hadn't fully recovered from this terrible shock by the time Van Buren came up for reelection in 1840. No surprise, then, that Van Buren lost the election. One could reasonably argue that he was defeated, as much as anything else, by his rigid adherence to the principles for which he'd always stood.