One thing that I have noticed within the Arminianism/Calvinism debate is that the concept of "Central Doctrine" is thrown around a lot, often pejoratively. The question is what is the main tenant which defines Arminianism or Calvinism? Such a question is actually quite relevant in any theological discussion, but I've seen it come up the most in A/C.
However, I think the concept can be answered in two quite distinct ways. I've differentiated these two ways of answering this by the terms 'distinctive doctrine' and 'core doctrine'.
A position's core doctrine is that belief or set of beliefs which are foundational for that position. It is the doctrine that every other doctrine within the system is based, and that idea about God, humanity, and the world that the position is attempting to protect. For instance, within Creationism, the core doctrine is the infallibility of Scripture. For Pentecostalism, it is the power of God being manifestable in the world.
It is important to note that this doctrine may not actually be distinct at all. Indeed, there are times when one's opponents hold to the exact same doctrine (as we will see in C&A). What defines a core doctrine isn't how unique it is, but how important it is.
For instance, in Calvinism one can identify two core doctrines. The first being God's sovereignty. Do Arminians believe in God's sovereignty? Of course, but that is what much of Calvinism is trying to protect. The second is monergism: only God is involved in salvation. Again, there are other versions of monergism out there, but protecting that idea is central in Calvinism.
For Arminianism, again, we can identify two. The first is God's goodness. Calvinists believe in God's goodness, but the Arminian is the one who is more intent on protecting that idea. The second would be the univerality of the atonement.
In both these cases, the second one I mention is based on the first. That's really more coincidence, but there you have it.
A distinctive doctrine, or defining doctrine, is that belief or set of beliefs that is unique to a particular position that allows you to identify it. They may not actually be the most important elements in the system, and may not be central to a person's spirituality at all (though sometimes they are). It is merely what differentiates it from other positions, like the Petrine authority of the Pope in Catholicism, or speaking in tongues being the initial evidence of baptism of the Holy Spirit for Pentecostals.
The distinctive doctrine of Calvinism is Unconditional Election (that's right, not determinism). If you believe in Unconditional Election, you're basically some form of Calvinist (for me, I would consider Amraldyianism a mild form of Calvinism). Everything in the Calvinist position either builds up the concept of Unconditional Election, or is necessarily concluded from it.
The distinctive doctrine of Arminianism is prevenient grace (that's right, not free will). If you believe in prevenient grace, you're basically some form of Arminian. Everything in the Arminian position either builds up the concept of prevenient grace, or is necessarily concluded from it.