Hurrah! Finally, a thought provoking thread!
On Morgoth's Powers
Melkor was originally percieved as the most powerful of the Ainur because he had a) a bit of every one of the Valar's powers and b) unlike the valars that could only understand some concepts of thought that Illuvatar planted in them, Melkor knew evil, fear and perhaps even what was good and pure. So the basis of Melkor's greatness was the nature of his powers and his innate knowledge.
However was he really that powerful? From the Silmarillion I always thought that Melkor was a jack of all trades but master of none. He could make, enchant and fightetc etc, but in each field he was not greater than the respective vala of which that talent was attributed to. He could never master the elements like Manwe or Ulmo, shape and create like Aule and Varda nor could he fight or hunt as well as Orome and Tulkas. He was versatile but I think that was the only advantage he had over the individual valars.
So is greatness measured by the innate ability of an individual or by deeds?
On Transformation
I do not really agree with Tolkien on the fact that by transforming one's self into flesh and bone, an Ainur would have greater power over the physical world. The world is made out of various elements in which most are not even part of the basic built-up of the living carbon-based body. I word I'm looking for here is linkage - I tend to think that if you transform into something, having the basic biological compatibility to an element you wish to control would help (why we are able to ingest small amounts of iron and still be healthy but Sulphur and heavy metals are a no go eg.).
Unless Morgoth's only aim was to control other living carbon-based lifeforms alone, I think he would have been better of in the "incorporeal" form. After all it would seem that the Valars had greater control and displays of power in a non-permanent body.
On Morgoth's Role in the Greater Plan
As Illuvatar stated itself, all sources of power were from him alone and those who thought otherwise were only deluding themselves. I should suppose the Morgoth's role was very similar to those of Loki, Surtur, the giants of Jotum - to lead to the creation of a new world. Of course Morgoth would not know but then who can understand the intents of an ominipotent deity?