It's actually as mentioned in Wesabe's postmortem (http://blog.precipice.org/why-wesabe-lost-to-mint/); for most people Mint doesn't actually get you to change your behavior. It just provides twentysomethings with some feeling that they're managing their finances, in return for getting pitched credit cards.
Harder and more worthwhile than UI or functionality fixes are changing twentysomethings' behavior, or doing something interesting for older people with more complicated financial situations. [Mint's useless for my parents, with multiple bank accounts, investments, retirement, education and health savings accounts, etc.]
One problem (and barrier to good competitors) are that aggregating finance data sucks. Mint started with Yodlee, which is nothing but a scraper for bank sites, and now runs on something Intuit built internally. It's hard to imagine many startups meaningfully tackling the problem.