How does taxation differ from common theft?

[Perdendosi]

Your response to number 3 is faulty. In fact, if enough people DO consent, you can legally rob me--enough meaning a majority of the state legislature to repeal the robbery statute or (within the bounds of the constitution) make it apply not to you or to the conduct you condone.

Of course, taxation differs from theft because the government--the same government that passes laws about what constitutes theft--engages in taxation. Taxation is definitionally excluded. Taxation is not theft because you have representation in the legislature to abolish/repeal/increase/decrease taxes. Even though you may not agree with the will of the majority regarding the tax, you had a chance to set the tax rate, what deductions you could have, and even what that money is spent on.
You don't get to negotiate with your thief how much they take. You don't (usually) get a voice to determine whether they take your TV, your DVD player, or your wallet. And you certainly don't get a voice in how those funds are spent and whether they can be spent for your personal benefit.

Finally, taxation is part of the social contract we've set up. To the extent that we live in a "Lockean" society where the power of government comes from the consent of the governed, and government's responsibility is more than just to keep us safe from invaders (Hobbes), we take collective action, we get collective benefit, AND we share the collective burdens, which include taxation. It's not just "legitimized," it's essential to have a democratic, stable society.

Regarding your first premise, note that it would also be very economically inefficient for individuals to pay for the societal structures put in place through taxation on a transaction-by-transaction basis. If we want to do business, but we need a road to move goods from one place to another, it would take WAY too long and cost WAY too much money for us to negotiate, create, and use the road. So, we could never do business. But if we put in a very small fraction of money away for roadbuilding, then a road will already be there when it comes time for me to sell my widgets to you. Without taxation, commerce would grind to a halt.
You say people can be found to pay "without resorting to threats of violence," but how? Because it's in their economic best interst? Rarely. And the transaction costs make it nearly impossible, as I've discussed before. By appealing to their emotions? Possibly, but that means you have to abandon the idea that people are basically rational economic actors, and that they look for self interest first (which are the foundations of capitalism) Even with taxes there is a need for charity, but charity could never build a fire department ("what are the odds that I will ever use it? It's not in my rational economic interest to pay lots of money for a fire department.")

Hope you like the counterargument. Or, at least, I hope it helps you to improve your points.

EDIT:
What is this "threat of violence" you speak of? The Government is not going to break your kneecaps if you don't pay by April 15; they may garnish your wages, reposess some of your assets, and incarcerate you, but that's a lot different than being forced to pay a mugger money or forfeit your life.