The Affordable Care Act isn't liberal enough. It should have been either Canadian-style single payer, British-style nationalized, or French-style combination of the two (France has the world's best health care system).
That said, the ACA is a huge improvement over the status quo. It will serve as a good foundation for more effective reform as we start tinkering with it.
There's a House bill going around right now to open Medicare to anyone who wants to buy into it at cost, the 'Medicare You Can Buy Into' bill. It would be a simple and effective public option.
As for arguments about the mandate...it was republicans who came up with the idea, way back in 1993, and it was included in the ACA by the democrats. After which, the republicans started bad-mouthing their own idea and claiming there weren't any republican ideas included in the ACA.
Anyway, if you're going to force the Insurance companies to cover anyone regardless of pre-existing conditions, you must insist everyone get into the coverage pool. Otherwise, people will wait until they specifically need insurance for some sickness or injury and *then* purchase insurance. So only already-sick people will keep insurance all the time, forcing the insurers to pay out more than they take in. This would make costs sky-rocket even more than they already are.
So if you outlaw discrimination for pre-existing conditions, you must have a mandate to keep the costs down. And if you're going to mandate insurance, you must have subsidies to help people pay for it. And if you're going to have subsidies, you must pay for them with tax increases and/or spending cuts elsewhere. And if...etc. All of these things are interconnected, so you must have all of them together.