When this show came out, it became immediately super popular, for several reasons: the cast (especially for Song Joon Ki and Song Hye Kyo), the setting, the pre-production. Basically, you couldn't even dare to try to find a flaw without risking to be virtually crucified.
In my opinion, it has many good points. Song Joon Ki and Song Hye Kyo have lots of chemistry (no wonder they became a real-life couple afterwards) and are simply perfect. They're also supported by an amazing cast. The setting (aka mostly the Greek island of Zante) is obviously beautiful and cinematography is quite good. All the characters are very much enjoyable. The bromance is adorable.
My biggest complaint, though, is the plot. The show drags itself out quite a lot, sometimes it's even openly boring. If you remove the special effects and all the aforementioned reasons, I agree that what you get is a lame story which, acted and directed by someone else, probably would have never get the same attention.
Overall, a good though overrated show. Nothing that will make you write home about.