[Kuruharan]

They had a common enemy. Whereas in European history a common enemy could be taken advantage of in order to leverage a particular feudal lord's personal gain relative to his king, Sauron could not be so leveraged, for obvious reasons.

Hmmm...potentially profound point here.

However, at the time when Gondor was at its peak, Sauron was nowhere to be found. After his fall at the end of the Second Age, Gondor didn't really have an arch-rival for a long, long time. I'm not sure how influential the potential presence of a common enemy could be.

The Lords of Gondor may swear fealty to the King (or Steward) and bring in their armies when it's called, but that doesn't mean its feudalism. To truly be feudalism the princedoms would have to have sovereignty (to an extent).

I'm almost certain that I saw somewhere that the Princes of Dol Amroth were virtually sovereign. Of course, I can't find it right now so maybe I dreamed it up...

I think Boromir88 has a good definition of what I was wondering about, however, there is one thing that I would add which I believe is the most important factor (at least from my perspective). Who is it that actually controls the military? Do the soldiers from areas away from Minas Tirith swear their loyalty to their local lord or do they swear to the King/Steward? (The issue of mercenaries could complicate things immensely, so I'll ignore it and focus on native troops). Pippin swore to the Steward, but he was essentially swearing from Minas Tirith and I assume that soldiers from the City and its attached lands (Ithilien and Anorien the "royal" fiefs) would swear likewise.

Potential parallel between England and Gondor: I also seem to remember reading somewhere that the Duneadan were a small class of rulers in Gondor over a much larger population of other peoples. This is similar to early Medieval England and the Normans and Saxons. However, I'm inclined to discount the importance of this in keeping the ruling class together. First of all, there is no indication of any tension of this nature (at least internally, the Kin-Strife was similar but external). Second, the Saxon population of England were no barrier at all to the Norman lords avidly thumping each other (see The Anarchy).