His unique aspect comes from his height, he is damn fecking tall indeed. There has been lots of tall strikers, yet not many as tall as him and producing so many goals regularly. In late years Zlatan is in that height bracket, but his atributes are way diff from Erling, in some aspects being more of a rare specimen than Erling.
It's not like Erling differs too much of the classic 9 big man. And in some aspects even having pace and power (like CR7 and R9) he preffers to avoid physical contact, instead of thriving on it like other players in the past in such role (Batistuta, Weah, etc) . So at the end of the day he has a fanstastic positioning (his best asset so far), great running into space with great pace and an great eye for the goal. But like I've said before he is more unique aspect is his height. BTW he has time to develope a better header to take advantage of it.
Messi is not that "unique" either regarding his style (Zico, Cryuff, best and touches of Diego just as examples or similar players) or the array of offensive output he can produce, he is unique because of the level in which he does those things and I do not mean it only regarding stats, but technical profficiency (virtuosism), combine with great pace and time and space awarness. That is what makes us put him in a conversation with Pele, Cryuff, Maradona and such fellas, not because there weren't players that combine such atributes before, or in his time, but they do it on a lesser level.
In any case, what sounded a bit odd on the original post that generated this conversation, it's that it's a lot more rare, unique in a sense of combination of atributes + level of execution to find a Messi, Pele, than an extraordinary profficient big forward or striker. His carreer might end being beyond extraordinary regarding numbers, no doubt about it.
Anyway today he scored in a way that suits him like a glove and City do not provide him as usual as it should.