Nadab and Abihu
Nadab and Abihu desired to offer sacrifices to God. Yet, they were not commanded to do so (Lev. 10:1) and therefore their sacrifices were unauthorized. In this incident, God decided to punish this sin by burning Nadab and Abihu to death by fire.
While it is true that the sacrifices were unauthorized, the reason why they so is because God did not command them to sacrifice in this manner. Thus, whatever is not commanded God told us we should not do.
All of these are in the Old Testament, yet in the New Testament, God did not change the principle of worship to the normative principle. In the same discourse with the Samaritan woman where Jesus tell us New Covenant worship is no more linked to physical location but rather to our "spiritual" location, "in Spirit and truth," Jesus told the woman that she and the Samaritans were not worshiping God correctly (Jn. 4:22). Worship is indeed greatly simplified (Col. 2:16-23), but it is here also that "self-made religion" (ἐθελοθρησκία, Col. 2:23) is proscribed, and thus the Regulative Principle re-asserted. One could almost say that the Regulative Principle for New Covenant times implies simplicity of worship.
The Regulative Principle and Elements of worship
The Regulative Principle states that we are to worship God only in the way He has commanded. What then are the elements of worship that God has commanded?
It is tempting, and it has been done in church history, to import Aristotelian philosophical categories when discussing what the elements of worship are. In both medieval and Reformed scholasticism, Aristotle's categories of substance and accidents were utilized in an attempt to discern what the elements of worship are and what elements are to be excluded. There has therefore been some churches historically that reject everything but the 150 Psalms of David and reject the use of all musical instruments (with the possible exception of one simple instrument to produce the base note in order that the psalms may be sung on the same key). Scripture of course knows nothing about Aristotle. Aristotelian metaphysics may or may not be helpful, but I do not think it is essential to embrace that in order to discern the elements of worship.
The important thing about Reformed worship is that it only embraces what God commands. Thus, the elements of worship include singing of "psalms, hymns and spiritual songs" (Col. 3:16), the preaching and proclamation of God's Word, prayer, and the sacraments. Notably absent are drama, skits, plays, movies, flashy strobe lights, and praise bands. Reformed worship is spartan in comparison to much of modern "worship," as we desire to do only the things which God command us to do. Yet, because it is to be done "in Spirit and truth," it should be truly meaningful without the false worldly impressions of glory given by flashy praise bands.