It's very unlikely that we'll be able to make any improvements to the Affordable Care Act until the democrats retake the House; the republican party has made it clear that they'll be putting all of their efforts into making the law fail, and have absolutely no interest in making it better. But on the off chance that the good guys retake the House and keep the Senate next year, what changes should be made to the ACA?
I think it's clear that the largest problem (aside from keeping for-profit insurers, of course) is the perverse incentive for companies to keep people under 30 hours per week in order to avoid having to either provide health insurance or pay the penalty. Given a functional Congress, this is an easy fix: rather than calculating the fine based on the number of full-time employees, calculate it based on the number of FTEs (full-time equivalents). With one simple change, it's now in a company's economic interest to have as many full-time employees as possible.
Another valid criticism, I think, is of lowering the limit on flexible spending accounts to $2500. In theory, with the better coverage that's now required, there's not as much need for FSA money. In practice, not only may you need to cover copays, etc, but FSAs can also be used for things that health insurance generally doesn't cover, such as dental. I unfortunately know from experience that even with dental insurance, it's easy to run up thousands of dollars in charges to fix just one or two teeth. Maybe we could actually start considering the mouth to be part of the body?
I'm sure there are others; you simply don't get something of this complexity completely right the first time. Within the basic structure of the ACA (that is, without getting rid of the insurance companies), what would you change?