Short answer: A lot. Short-term, limited-duration insurance is not required to follow the ACA’s core consumer protections: it can deny you or charge more for a pre-existing condition, leave out essential health benefits like maternity, mental health, or prescription drugs, and cap how much it will pay. It also does not count as minimum essential coverage.
Short-term plans are designed as a temporary stopgap, not comprehensive coverage. By definition, short-term, limited-duration insurance means health insurance coverage provided pursuant to a policy that meets specific federal conditions, and it sits outside the individual-market rules that protect Marketplace plans.
Because of that, short-term coverage is not subject to the prohibitions on discrimination based on health status, pre-existing condition exclusions, and lifetime and annual dollar limits on essential health benefits. In practice that means a short-term insurer can turn you down or exclude care for conditions you already have, skip benefits a Marketplace plan must include, and place a dollar cap on what it pays. Federal rules now require these plans to display a clear notice of their limits, and enrolling in one does not satisfy any coverage requirement or qualify you for premium tax credits.