Oh, Superman. Love him or hate him, you have to acknowledge his ubiquity and the cultural impact he's had. But there's one thing that almost everyone seems to agree on: his alter ego disguise is lazy. Surely no one expects us to believe that slipping on some glasses will disguise a super-buff, extremely distinguishable alien. Now shush, as I totally show you why that's wrong.
This is a drawing from Frank Quietly from All Star Superman fame. It's a diagram that shows the major differences between Clark Kent and Superman. The main difference, obviously, is the posture. Kent is slouching with his neck craned. Throw a loose-fitting coat over him and he would appear more pudgy than inhumanly strong. Superman appears more muscular and carries more confidence than Kent's apparent apathy. This also carries over to their facial expressions as well. Next is the head. Kent's hair is more matted and messy. Superman's is more slicked back. And then, yes, there are the glasses.
But wait, I've got a bit more to add. Superman has a name, Kal-El. Most characters are aware of this, but this is actually a pretty important point because it's not like people are looking for Superman's ego. As far as they know, he doesn't have one. He's a god, why would he even pretend to be human? It's never crossed their minds that he might be doing just that.
Another point is Kent's mannerism drastically differing from those of Superman's. In many iterations, he is seen as extremely clumsy. Tripping over things, dropping things, etc. In some shows/movies, Clark even speaks with a higher pitch than Superman. Superman, on the other hand, is a humble, yet (again) self-assured, individual. You would never see Superman trip over someone's luggage.
In my opinion, all of this fits well with the idea of how Superman is viewed. Superman is a symbol. No one is looking for a person behind the symbol because they believe the symbol is all he is. Anyone familiar with the character knows this isn't the case. They'll know the farmer's boy is an important part of the character.
To wrap things up: the posture and facial structure of Superman and Clark Kent differ greatly, with Superman have a more confident appearance overall while Clark Kent looks pudgy and kind of average joe; no one expects Superman to have a human alias at all; and their mannerisms are worlds apart.
And that about does it. There's no doubt that Superman's disguise is silly and this isn't helped when artists draw Superman and Kent don't even bother to use the differences I listed (remember, there are many artists in the comic book industry, not all will think of including touches like this). Nevertheless, I think this is a decent defense of the classic dilemma.
As your reward for reading brief, yet niche, article, here's a funny moment relating to Superman's identity from the Animated Series.
