@boiga:
Good comments Aohige, Panda and Jacoobus.
Those of us who have been around for far too long remember this exact same discussion back in Enies Lobby. Everyone wanted Sanji to start launching Rankyaku's. Zoro's damage resistance was determined as proto Tekkai, and the Alabasta clock tower seen was obviously some kind of geppou jump. Heck some posters still like the idea of Life Return being a major part of Luffy's repertoire.
I agree with Jacoobus in saying that when a new technique or style is introduced, any previously hinted examples need to have the exact same results and preferably similar graphic illustration, or you can't assume they are related. This is especially true for old techniques that are already explained in the context for which they are introduced.
That last paragraph is something I can definitely get behind, and is in fact I think something that many of people who use Mantra being Haki as support for other things, such as Zoro's fight against Mr. 1, miss. While I said before that it lends support to such arguments, and stand by that, it is nonetheless a distinctly unique case.
To wit, as mentioned multiple times now: Mantra and CoO Haki are and were, aside from name, totally indistinguishable. For my part, at least, when Sandersonia called Luffy's attacks ahead of time and proceeded to dodge them, my first thought was that it was strange that she had Mantra, which I had assumed to be a Skypiean thing. Therefore, when it was ultimately revealed that the two abilities were in fact one and the same, it was really nothing more than confirmation of the obvious.
On the other hand, there is… well, everything else. Of course, I can see it being tempting to believe that things such as in Zoro's fight with Mr. 1 are explained by Haki, and I would not say that they are baseless arguments. However, the fact remains that, if they are indeed explained by Haki, that fact is not immediately apparent, and would involve more depth to Haki than we have yet been shown. On that note, and playing devil's advocate to the Zoro as Haki argument, it's worth noting that, despite how all the theorizing would make it seem, Haki as confirmed so far has in fact been quite straightforward. CoC has been used to K.O fodder, CoO to predict people's actions, and CoA to create stronger attacks, tougher defense, and hit logias.
All that said, I think it might be important when talking about these things to distinguish between Haki, the ability, with a capital 'K', and willpower. The two are undeniably and explicitly intertwined, the former being a sort of expression and manifestation of the latter, but they are not the same thing. With that in mind, I feel a worthwhile third possibility would be unexplained things such as in Zoro's fight are also derived from willpower -- or spirit, or whatever other inner force you wish to call it -- but simply in a different way from Haki, and thus continue to stand on their own as unique abilities.