I think he is more knowledgeable and tactically astute than Ferguson, for example. He also has excellent man-management skills. I don't think he has got the hunger of a Ferguson though. Ferguson will go down as an all time great, however distasteful that may sound. I think Hiddink is more talented than Ferguson in purely footballing terms. But obviously other factors come into it such as personal attributes (hunger, desire, ambition etc) and this is where he falls short. So, in that sense I think you are correct - and of course being a great manager isn't just about one or two attributes - but, I think he has got the natural talent, knowledge and tactical acumen to have achieved truly great things. I think winning the European Cup with PSV in 1988, their only E Cup triumph (albeit on penalties), and securing the treble in the process, is indicative of this.
Due to personality flaws and choices, he hasn't achieved what his ability warrants and what some inferior managers have.
I guess, you could also say this about plenty of footballers who had the talent to be truly great and remembered as such, but fell short in the personality aspect.
We did catch a glimpse of what Hiddink could do in his short spell as Chelsea manager, when he was able to make an instant impact, galvanize and unite the players.
In years gone by, I would've been happy with him as LFC manager; but he would only have been a short term option, as he could've done great things over a period of 3 years; but don't think he has the will to do this for 5-10 years.
I guess, to prove yourself as an all time great, longevity or sustained period of success at a club is also important. This is where he falls short and I think associating the word "potentially" is fair.
I think his talent is beyond doubt, though.