Hackett takes a different angle. He says "you can't pay a soldier to believe". It's about Shepard's sense of hope and resilience. Not power. At least he does that for me. It made sense to me, when he mentioned Akuze/Sole Survivor origin. Which is more like Ripley in Aliens. Someone with a strong will to live. Not a necessarily powerful person.
The interesting thing with Liara, who used to believe the most, is that she's faltering more in ME3. She entertains the idea of the cycle ending and doesn't know how Shepard manages any of this, and was even originally written as possibly committing suicide after Thessia (of course, they didn't do that.. but it shows how far they were willing to take her new direction).
I agree ... sort of ... Even Hackett admits that there is just something special about Shepard. His/Her charisma, strength, hope, resilience, self belief, prowess, etc. It's that certain ... "something" ... that draws people to Shepard and gives Shepard that "Aura". It's why I used Nyreen's dialogue as an example, although she is talking about her relationship with Aria, even Aria succumbs to Shepard's "power".
I don't think there is an actual word that can encompass it all, so I use the word power, in inverted commas, to define the undefinable.
It's a recurring theme throughout ME3 at least, and to a lesser degree ME1 and 2. Tali stating that she only took the position of Admiral because it's "what Shepard would do" Liara's frequent "I don't know how you do it" speeches. Traynor, Dr Brynn Cole, Jack, EDI and many more, including even Garrus (the Dossier on Hagalaz mentions that his exceptional leadership ability will always be overshadowed, while he's in Shepard's company) all referring, directly or indirectly, to Shepard's "power" (there's that inadequate word again)
They almost, and in some ways actually do, deify Shepard. But really, form a purely pragmatic gameplay point of view, it's a smart move by BW. Who doesn't want their avatar, the one they have spent hours, days (weeks? months? ... years?) controlling, to be worshiped as a bada$$ demigod?
And yeah, well, as for Liara? I don't think the writers could quite decide who they wanted her to be, or at least what sort of 'person' they wanted her to be. As the "Shadow Broker" she seemed quite strong and solid, very pragmatic and mature sometimes, but whenever she's around Shepard, she turns back into the "little lost girl" again. Not sure if that was "planned' or just a case of bad writing, in the sense that they didn't give her character any real progression or "growth".