[Boromir88]

I would say it is largely because of the various evidence that the Nazgul DID NOT have added physical strength prior to this scene that the breaking of the Gates (at least partly carried out with the use of a spell) seems noteworthy to me.~CSteefel

Prior to Pelennor Fields there was rarely a purpose for displaying such force by the Witch-King.

In the first two books the Witch-King is not out trying to destroy the Shire, or even kill anyone. So, why would he need such a power as breaking down gates and lighting up his sword? He had to conceal his power because he was looking for information, as busting through The Shire's hedge and literally scaring people to death (as what almost happened to Grima) would have been counter productive to his task.

This doesn't mean before Pelennor he was incapable of such a force, just that such a force was not needed because of his task of hunting down the Ring. And in his way stands 4 hobbits with a ranger, who's courage he greatly underestimated:

Escaping a wound that would have been as deadly to him as the Mordor -knife to Frodo (as was proved at the end), he withdrew and hid for a while, out of doubt and fear both of Aragorn and especially of Frodo...
But above all the timid and terrified Beared had resisted him, had dared to strike at him with an enchanted swords made by his own enemies long ago for his destruction. Narrowly it had missed him. How he had come by it - save in the barrows of Cardolan. Then he was in some way mightier than the barrow wight; and he called on Elbereth, a name of terror to the nazgul. He was then in league with the High Elves of the Havens.~The Marquette Letter (Found in Hammond and Scull's Reader's Companion)

Why don't we see some sparks with the Witch-King in his meeting with Frodo at Weathertop? Probably because he doesn't have a powerful Maiar lighting up the sky that is opposing him. He's got some hobbits (Merry and Pippin who had fallen down in fear and Sam who had shrunk and whined beside Frodo, Frodo himself who was terrified, and Aragorn who was at this time MIA). The Witch-King just didn't think that Frodo would overcome his fear and strike at him with an enchanted blade that was as crippling to him as the morgul blade was to Frodo...so the Witch-King, as I would have done had someone tried to strike me with a blade that would completely cripple me, high-tailed it out of there.

Now how is the Pelennor Witch-King different (as far as in terms of 'power') different from that of the Witch-King earlier? There isn't a difference...several times he was referred to as a 'sorcerer' before we see him at Pelennor:

"For even the Wise might fear to withstand the Nine, when they are gathered together under their fell chieftain. A great king and sorcerer he was of old, and now he wields a deadly fear."~The Council of Elrond

Those who used the nine rings became might in their day, kings, sorcerers, and warriors of old.~The Silmarillion; Of the Rings of Power and the Third Age

King of Angmar long ago Sorcerer, Ringwraith, Lord of the Nazgul, a spear of terror in the hand of Sauron, shadow of despair.~(Gandalf to Pippin) The Siege of Gondor

So, clearly the Witch-King must have done some sort of 'gate-busting' sorcery before Pelennor Fields, or he would not have been known to people as being a sorcerer.

We see his display of power in his attack against Gandalf on Weathertop. The Wraiths wait for night where their power would be greater, both Aragorn (when he arrives at Weathertop) and Gandalf say this was no walk in the park:

"But they closed round at night, and I was besieged on the hill-top, in the old ring of Amon Sul. I was hard put to it indeed: such a light and flame cannot have been seen on Weathertop since the war-beacons of old."
"At sunrise I escaped and fled towards the north. I could not hope to do more..."~The Council of Elrond

It is Gandalf who has to high-tail it out of there this time, he is the one that has to hold them off and he gets away. If someone has a gun are you going to go charging them with a knife? If Gandalf is lighting up the sky, the Witch-King (and the other ringwraiths) were 'sorcerers' because of the possession of their rings, and Gandalf even has to flee, put it all together.

Where does the Witch-King have 'great physical power' at Pelennor Fields? He still has the boldest of Gondor running in terror. In fact only the Knights of Dol Amroth and Gandalf were said to have withstood the Nazgul. When the Witch-King barges through the gate it is only Gandalf that blocks his way. And the Witch-King, seeing Gandalf was standing in his way completely unafraid, leaves that situation to go tend to another.