What ever you guys here on the PB tell me it is. Duh! 
Just kidding.
Those who believe in the Sovereignty of God, the 5 Solas, the 5 points of Calvinism to start with and have a desire to grow and learn more of the things of God. Not easy believe-ism as taught in many modern churches. Also to some point the desire to worship God in the way that He chooses for us to worship Him not any ole way we please to. Most of all Reformation is a condition of ones heart as well as outward performance. So I guess I would say that "Reformed" means being put in line with the Word of God and all that it teaches.
Really though
I would suggest more precise thinking on what you/we really mean by "Reformed" - for the sake of our own understanding, and also for the sake of fruitful discussion with people unfamiliar with aspects of Reformed theology with whom we have opportunities to talk about things.
Here's what I mean: In your post above, you essentially listed as the qualities of being "Reformed" as: 1) divine sovereignty, 2) five Solas, 3) TULIP, 4) a desire to grow in the Lord, 5) a rejection of easy-believism, and 6) at least some recognition of the Regulative Principle. You then (rightly) noted that all of those issues involve the heart just as much as the mind and actions, and summarized them by noting (again, rightly) that they are all in-line with the Word of God.
Now, are those six things you noted all essential elements of Reformed theology? Absolutely! But nothing particularly sets them apart from many other issues that are equally vital to Reformed theology: Some examples are Covenant Theology; the Creator-creature distinction; a rejection of hyper-individualism and a recognition of the central role of the Church in God's plan for His people; the full sufficiency of Scripture for the edification and spiritual knowledge of believers; the centrality of the Cross and the Gospel; a recognition of the value of remembering the wisdom of past generations of believers; God's glory as the highest priority of everything...
I could go on. But the point is that no "list" of a few doctrines or emphases will ever be able to encompass the either the "core" or the "distinctives" of Reformed theology. That is because it is a comprehensive worldview coming from the whole of Scripture, and as can be seen from the various Reformed confessional standards, they don't emphasize, say, the doctrines of grace, or Lordship Salvation, or the Regulative Principle, or other things about worship, any more than it emphasizes other particular elements. To be sure,
compared to more foundational doctrines, there are going to be some issues that are more "minor" in application and substance, such as lapsarianism, common grace, millennial views, and theonomy. But at the same time, we will never be able to give an "A, B, C, D..." list and present it to people as the "necessary components" of Reformed theology. I imagine one reason we're all tempted at times to do so is because different doctrines are under more attack than others during different centuries and generations. For instance, until the Synod of Dort, no Reformed church or theologian knew anything of "five points." To be sure, they are effective in explaining to people the Reformed understanding of who does what in salvation, but originally, they only came to be emphasized as a group since they were the five chief doctrines that the Arminians challenged out of the Reformed system of theology as a whole.
You also (again, rightly) identified a piety of heart and a desire to grow in the Lord as a chief and necessary component of Reformed theology, and also pointed out that we indeed understand Reformed theology to ultimately be the natural result of "being put in line with the Word of God and all that it teaches." But the problem with
defining it by either of those two criteria is that
all evangelical Christians have a desire to grow closer to God, and likewise believe their theology to be most in-line with the Word of God.
So if it's not inclusive enough to explain Reformed theology by the six-part list you initially described, and yet if we have to be more specific than simply saying "the full teaching of the Word of God," what
is a good path to take in explaining Reformed theology to people? I would say going to our confessions. Why? I try to describe the basis for that
here.
