First of all when answering 'what is
Tom Bombadil' you have to see it from a broader context. Since he was first,
the Master, did not get affected by
the Ring, is a
mystery,... you have to go back to the creation of
Arda. The creation was made by a song, by the
Valar. Pick one of those to start with, because all lesser (even
Maiar) would be under effect of the Ring. The Master... leaves us with one person:
Aule.
Second, if he is Aule, and he is such a fine and wonderful
god, why doesn't he choose to be more helpful? Put another way, why isn't there power in him to fight the enemy? The answer to this question is simpler than one might at first imagine...
It is the song of creation, I believe, that contains the conflicting instructions both
Ulmo and Osse are following, different parts, elements, or themes of the whole. If I am correct, then Ulmo's power to help the
Elves is both limited by and partially determined by the
Music of the Ainur, insofar as it established the existence of the earth and determined its major
events...
While Ulmo may have had free will as he sang his part of the song in those distant times, he is now bound by what he sang and cannot go beyond or change his part. If
Tom is Aule, then he too is bound by his part in the song and although sympathetic and concerned, he can only help the
Hobbits and the Free
Peoples of the West in little ways.
This is important because if Tom is Aule, he may not be allowed to do anything what could change the balance of good and
evil in
middle earth without destroying the harmony. Even if it sounds strange, but the balance of good an evil - or high tones and dark tones - is important for a sound. And
Sauron as a dark tone had his counterpart in the good forces of
Saruman and
Gandalf. An indication for that is, that Gandalf became more powerfull after Saruman joined the dark side.
I don't say Gandalf could match Sauron face to face. No way. But his power lay in his popularity in middle earth. He was loved of many good people and could motivate them to fight the evil - that was the real power he had. And his hope lie in the hearts of
men, because they could switch their minds from bad to good while elves and orks belonged fixed to that part of harmony they were made for. If I am right, the balance depended on a few good or bad men and could switch easy and was very fragile. A huge good power would have desruped the whole harmony.
Its just like making sweet-sour cabage. Too much acid is horrible, too much sugar, too. And if you nearly hit the point of balance and is only slightly too sour, you wouldn't put a suggarblock into but a few grains.
Another indication could be that Gandalf left middle earth after Sauron was defeated and Sarumans death. You can have short times with just having high tones or dark tones in a melody, but it never ends without the full harmonic spectrum.