I am no lawyer and no expert on Obamacare. Perhaps the tax argument was just fine in this case. I'll defer to others on that.
The more obvious argument to me in the lead up to this ruling was to say that Congress unambiguously has the authority to spend for the general welfare (and it's doing a lot of that with this reform), and it unambiguously has the authority to do anything "necessary and proper" to accomplish anything that it has the authority to do under Article 1 Section 8. You could make a good argument that the mandate is "necessary" and "proper" for carrying out its spending for the general welfare. Ergo, it's perfectly constitutional.