What is liturgical worship? I think we have to start with the recognition that God is not safe. As Lewis so famously put it "Aslan is not a tame lion." We die apart from Him, for He alone is life, but He's like electricity. He can fry us when handled carelessly. And there is nothing more careless than ignoring HIS instructions and dreaming up some notions of our own and thinking they should serve just fine. Try that out on Uzzah, or Nadab and Abihu. Israel dreamed up her golden calf and thought it should work as well as any other way. God zaps the lot. He's not tame.
But HE is life and He is loving, and so HE provides the way for us to approach Him and live in His presence and to find that presence to be the very gift of everlasting life. And the way He provides is always through the Lamb. Find the Lamb on the altar and you've found access, a way to live in the presence of Him who is all-holy in such a way that His holiness wipes out your sin without wiping you out with it. Find the Lamb. That's the key - all the way from Eden, where the garments that Adam and Eve made up for themselves wouldn't do, and GOD clothed them with skins; to Abel's offering that was accepted; to the Passover and the Scapegoat, and the daily Lambs at the temple; everywhere you look through the OT it was all about the Lamb.
So when John points to Him and says: "Looky there! The Lamb of God!" He was saying: "There's the access to the Holy One who will be your life; there the untamable God, the Live Wire, comes down to you to give you Himself in such a way that you won't be destroyed, but filled with Light!
Find the altar where the Lamb is, where His blood is poured out yet (into mouths) and you've found liturgical worship. I'd posit that liturgical worship from start to finish is all about the access to God provided via the Lamb. It's not our dreamed up way of offering stuff to God; it's God's own appointed way of giving life, forgiveness, HIMSELF to us. "Open your mouth wide, and I will fill it" He declares (Psalm 81). Or said another way: "Take and eat, this is my Body."
Oh, and let me add that it is my belief that nonliturgical worship is worship where the Lamb on the altar in His completed sacrifice of love is not the center - but something else is. And it doesn't much matter WHAT that other thing is, if it's not the Lamb of God who takes away the sin of the world and dishes out His peace with His blood, you've got something other than liturgical worship happening.