Certainly, part of what makes Gandalf compelling is the idea that "still waters run deep" with him. Outwardly, he doesn't appear that wise or powerful ... just a guy who likes hanging out with hobbits more than he should and smoking his pipe. A lot of his more interesting adventures happen outside the main narrative, so we know he's a badass, but have to wonder at the specifics a lot of the time. But at the same time, Tolkien gives us flashes of his power—both in terms of his actions (summoning eagles, fighting Balrogs), and in the way some of the seemingly important characters in the story defer to him. Elrond and Aragorn are kings, and yet they both seem willing to put their trust in the seemingly humble wizard. It draws you in—who is this guy, and why is he the one everyone turns to?I would put this somewhat differently. It is through Gandalf that hobbits, i.e., ordinary Englishmen, enter into epic magical stories. Gandalf travels out from the Shire and does amazing things somewhere, then comes back, again and again. Those of little depth or curiosity know him for his spectacular fireworks shows. But for those who understand, his friendship is a gateway to that vast plain where good and evil battle for the universe, magic is real, and heroes overcome.
But I think there's a more personal connection which might be the more powerful one if you're inclined to feel it. Although Middle Earth has a race of men, the hobbits are in some ways closest to us as readers. "Men" in the Tolkien world are mostly armies moving around battlefields, though there's a few such as Aragorn who rise to be more than that. Hobbits are the simple people who live their lives and try to be happy and do what good they can. They're meant to be us.
And this is what makes it personal: Gandalf's faith in the goodness and competence and hidden abilities of the hobbits—even when the powerful and mighty like Saruman and Thorin Oakenshield doubt their worth—is Tolkien's faith in the goodness and competence of normal people, of you the reader. Through Gandalf, Tolkien is saying he trusts you and I to do good, to live our lives well, to put happiness and friendship above material gain, and to fight evil as best we can, even if we don't think we have the tools at our disposal. . . . His faith in the hobbits is his faith in us.
I would also say that while Gandalf several times says things that praise all hobbits, and he seems to like them, he is rather particular in which ones he invites to go along on his adventures. He is the means by which Tolkien's universe recognizes which apparently ordinary folk are capable of great deeds and calls them to their destinies. So when he, to use Macdonald's terms, shows faith in us the reader, it is not just a generalized confidence in ordinary people, it allows us to fantasize that we might be the ones called to greatness.
Another facet of Gandalf's power, to which MacDonald alludes, is that Gandalf's most important magic is not in doing things but in knowing and understanding things. His most important contribution is to understand the global forces behind what seem like disconnected events to others, and to see through the lies of the enemy. He knows what needs to be done. In a crisis situation, that can be the most compelling magic.