Thing is, Zoro is awesome without any effort. He is the genius of awesome. He is awesome when doing something as simple as going to a shop to buy a couple of swords, and he is awesome while taking a beating from Kuma.
He just needs to stand there doing nothin', and he is awesome. xD It's not just "because", but it's just what the character was built to be. Everything he does and says is gold. Because Oda decided that's the kind of guy he should be.
The most likely reason someone might not care for him, is because he is… kinda typical. The standart shonen cool guy, that is perfect in almost every way but happens to have one major fault. Like Shin from Eyeshield 21, he was the perfect hard working genius, but he was useless at handling tech, breaking GPS's and computers by just holding them.
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Thankfully I still find those types of characters hilarious when they do something retarded.