This song is about disappointment, or being "let down", basically.  The general concept is simple, but some of the supporting details are kind of nebulous.  The first few lines of the first verse are especially so:

transport.  motorways and tramlines.  starting and then stopping.  taking off and landing.

Thom's lyrics in this song, as well as others like "Fake Plastic Trees", achieve everything that imagist poetry failed to do in its day.  He is portraying an image of a bustling metropolis, with various modes of transportation travelling within.  This image suggests a routineness, even drabness, to life.

The lyrics continue:

disappointing people. clinging onto bottles.  when it comes its so so.  disappointing.

Thom recalls a night that inspired this song.  He was sitting in a bar, and drunkenly found amusement in the idea of the floor falling out, and the people at the bar being suspended only by their bottles of drink.  This lyric first mentions disappointment in people, presumably in whom faith had been trusted.  It continues to detail how these people deal with their own disappointment.  The last two lines can be interpreted two different ways.  One is "When it comes it's so, so disappointing", hence "very disappointing." 
On the other hand, it could be read "When it comes it's so-so...disappointing."  This suggest a struggle to win a prize.  However, 
even when the goal is achieved, the prize is far less glorious than it appeared from afar.

The chorus: "Let down and hanging around.  Crushed like a bug in the ground." Fairly self explanatory.  The first line could be taken literally according to the bar-story inspiration, with the people "hanging" "down", but on a more general level, it is a depiction of the devastating feeling of disappointment.

In the next verse, the lyrics describe the bug annihilated only the chorus prior.

shell smashed.  juices flowing.  wings twitch.  legs are going.

A wonderfully bitter image.  "Don't get sentimental, it always ends up drivel."  This further shows the speaker's bitterness, demonstrating a loss of faith in the sincerity of the emotions of others.

In the second half of the verse, he declares that he will not be one of these "disappointing people".  He proclaims, "I'm gonna do something really incredible."  But this statement is insincere, and he knows it.  He begins by saying "One day...", which sounds unusually similar to "I'll never get around to it."  By reducing his plan to technicalities, "a chemical reaction", he destroys the glory of the dream.  And finally he realizes, whether he is right or wrong, that he will be "hysterical and useless".  That is, not only 
will he make a fool out of himself, but probably nothing will come from his humiliation.

The final verse is the most tragic.  He states, "You'll know where you are with..." and trails off.  The lyrics then read, "Floor collapses, floating, bouncing back, and one day, you'll know where you are."  This suggests that the subject's world will be capsized, but they will recover.  However, this recovery will be at the expense of the subject's dreams.  Very much like 
"Comfortably Numb" by Pink Floyd.  "You'll know where you are with..." implies "You'll know where with your life."  He is saying, "You will accept the disappointment that is your life.  You will no longer be disappointed, because your expectations will be set at zero.  And that is what you shall receive."

At face value, its a complaint of mass transit.  But upon a closer inspection it turns out to be a sad (though accurate, or at least so I believe) critique of modern society.  The planes, the trains, and all the other gizmos are simply ways of getting us from here to there, but with out any real destination.  It tells of being alienated in a world that really doesn't give a shit about anyone (or anything for that matter).  Before people had purpose (or at least thats the romanticized-nostalgia mentality so popular since who-knows-when would have us believe), but now a days everyone seems to be running in circles, never achieving a thing.  You're born, you're instilled a set of values society deems necessary at the time, you are expected to transmit these values onto the next generation, and then you die.  Pretty bleak, but not completely.  For you see, there is another side to the song.  Sure, it speaks of dissappointment (the lyrics express so quite clearly), yet, it also speaks of hope.  "One day I am gonna grow wing", one day we'll be able to get over this mess, one day we'll know just what the fuck is going on.  Well, maybe not entirely, but there is certainly a lighter side, and perhaps the words don't say so as much as the actual music.  The very last segment (as the track morphs into Karma Police) has an up-lifting effect.  It posesses a distinct soothing quality that can't be described, it must be experienced, again, and again, and again.  Even if the world seems like a barren and hopeless place, by the end you know that somehow it will all work out.  That one day you'll be looking back and laughing at all those tears.  I really can't think of a better song to listen to when you're sad.

By Evan Roskos

Let Down is another case where people want to throw album's entire theme into one song. Thom's explanation is a fitting place to start (the more famous explanation it seems). The whole bar scene, where he imagined everyone clinging to their bottles. But basically it's about life not turning out the way one imagines. the regulation of life has crushed everyone. The first stanza, note, is about manmade transportation, while the third is about bug transportation. and bugs are so hard to kill when they can fly. But ultimately they are easily crushed.

By Steven Tramell

Such a great song, and a perfect example of thoms great writing ablility. In the same vein as 'fake plastic trees' this song starts off soft and soothing, then builds in power as it progresses. This just climaxes the words and meaning near the end of the song, when he harmonizes with himself "one day, i am gonna grow wings.." The words paint the bleak landscape of human life (as most on this album) and like most on the album, it does not leave the listener devoid of all hope. Even when viewing the world as it is, even when realizing the things would move just as machine-like without any one of us, thom never lets it die at that. The entire second verse is of the death of a bug, then goes into the chorus "let down and hanging around, crushed like a bug in the ground.." the most moving part of the song is at the end, jonny is really on some weird time signature, and you just get the feeling of power from the words "one day, i am gonna grow wings, a chemical reaction, hysterical but useless, hysterical but.." and thom is hitting the falsetto singing "where you are with, where you are.." in short, i think the song is about trying to find hope, goals, and meaning in a life and world that is very self-sufficient. in the face of technology, we have little control over how everything works. if the power shut down, could we get it back working? i couldn't. thom does a wonderful job expressing his emotions about this kind of dependence we have. 'let down' is a beautiful song.

By Tina Jeong

Although there are many problems in life, it helps to talk to others. In order to maintain some form of identity and sanity, one tries to find someone to confide in. When this is attempted, it could go either help or not but in this case, it fails terribly. Not only does this blow one's mind away, it brings the person back into reality and shows the pessimistic view of life--how there is no guarantee of security and friendship. This is my interpretation.

By Mark M

Let Down is the one song on ok computer that always makes me cry. its another one of those "life sux, here is what is wrong with everything, and here is how i will make it go away" messages, but its done so beautifully. the main theme is being disgruntled and defeated by day-to-day middle class, working class existence. always in line, 2 steps behind, in the cubical. never feeling special individual or beautiful. its a distant brother of fake plastic trees, that idea of hating the facade of modern living, but both songs feature that moment when the spirit breaks out and gets away. when thom hits that high note with " you'll know where you are" and it overlaps with "one day i'm going to grow wings", you just see hope creeping into the room. just like when thom sang "but i can't help the feeling, i could blow through the ceiling", you feel it, deep down, that need to escape the life you're wasting away in.

The crushing of the bug for me is a metaphor for when held back emotions DO come out.

"Shell Smashed. Juices Flowing."
Letting down, if you will, the protective strong outer facade everyone trys to put on, and delving into the jucy innards.

"Don't get sentimental. It always ends up drivel."
Trying to convince onself to stop the tears, something we all do because no one wants to be seen as "weak" or "a crybaby".

"floor collapses floating"
While crying, in a confused distraught state. But then

"bouncing back and one day you'll know where you are."
The release of emotions is healthy and good in the end, as we're being honest about ourselves and our feelings and we'll stop telling ourselves that everything's OK. We'll then "know where we are" and face our problems and bounce back.

To me, the SOUND of the song suggests to me that it could never ever be depressing. Sure it's dissing on society like the rest of the album, but it's showing an instance where we're not hiding our feelings and uptight and constricted. A beautiful instance portrayed with a beautiful song.