There are a variety of reasons why the ACA is a bad law. Primarily, it is because it leaves intact the insurance companies and the outrageous costs of medical procedures, and does not put an upper bound on medical malpractice awards. Basically, it just guarantees the insurance companies a few million additional subscribers, and puts the onus on employers to suck up the cost difference (either through their company contribution or the penalty they pay to have employees shop the exchanges on their own). And don't get me started on all the fees. And how about the fact that we're still getting final regulations from the executive branch? Congress enacted a law over which they gave up control to the executive? Well, they maintain the ability to stop funding, sort of; but that isn't going to happen.
tl;dr: I'm mostly just against the individual mandate -- which is little more than an admission by the government that they couldn't find another way to fund the law. But don't underestimate the impact of all the fees on companies. And the unintended fees in the form of the costs of implementing the various reporting requirements (e.g., 30 hour; 6055/6056).