If it wasn't for Jun Ji Hyun lead role interpretation, this drama would have been nowhere that good. In all honesty, while pretty close I'm not entirely sure this drama deserve its 8.8 rating (at the time I'm writing this) but since it brings up a lot of passion and interest (from what I've heard), maybe that's enough in itself to justify it.
As for me, this drama success is clearly related to Jun Ji Hyun : she depicts a character borderline lunatic, superficially arrogant and ... amazingly funny to watch. Not that Kim Soo Hyun is not any good (he's actually quite subtle and believable as an alien), but to me the alien here in this show was clearly Jun Ji Hyun.  
While the romance seems the main focus (and is quite good at that), comedic scenes and situations rhythmed the drama so well that there is not so much room to loose interest. And again, the exact moment where you understand that Jun Ji Hyun 'Chun Song Yi' character is not exactly the superb arrogant idiot she seems, you're screwed. Jun Ji Hyun is just imperial in this drama and get you hooked - line, sinker and all - in no time.
Still, there are inconsistent details and poor choices too. There are too many to point them all out, but at the same time, of too little importance to take them seriously. This is not actually bringing the show down, since at best it gets a little annoying from time to time.
So in the end, what remains is an overall very well-written drama, asking interesting questions, answering cold hearts with warm feelings, and along the way providing some action to entertain the malicious villain (who is by the way quite good at being a villain, thank you Shin Sung Rok).
Maybe that's enough to justify the 8.8 rating, after all ? That's not for me to decide. You go watch this then you tell us.