There are two important threads running through In Rainbows that I feel are the key to appreciating it:
1) This is Radiohead's most human album, full of songs about love, worrying about death, and wondering how it all adds up.
2) This is Radiohead's most direct and concise album, full of songs that are road tested, tight, and immediately engaging.
Due to both of these, In Rainbows can initially strike a listener as being...too easy. Radiohead albums are supposed to be something that you struggle with and have to think a lot about, aren't they?? They're supposed to grow on you and open up new layers and meaning with time, aren't they??
In Rainbows commits the "crime" of being an accessible, immediately enjoyable Radiohead album. Patterned after the lean-and-mean classic rock albums of old that barely scraped the 35 minute mark, In Rainbows is the sound of a band accepting that they might not have any place left to go, any great experiments to undertake, aside from the least obvious one: to make a great pop album. Which is exactly what this album is. Lyrically and musically, it is Radiohead's pop album.
Though, to be fair, it's still really artsy and interesting, songs full of small delights and surprises.
If Hail To The Thief was a long, semi-sprawling album that collected and perfected all of the sounds that the band had explored up to that point, then In Rainbows is the band boiling down everything that makes them great until what's left in the bottom of the cooking pot is 40 minutes of perfection. What's most surprising about the music on In Rainbows is the revealing of the emotional, human heart that's been at the heart of Radiohead songs all along. Kid A was pretty abstract, but for music that was often criticized for being withdrawn, intellectual, and cold, using the Rosetta's Stone of In Rainbows it begins to make more sense. At any rate, Thom Yorke remarked that most of this album was his attempt at "seduction songs", which helps explain why In Rainbows sounds so romantic, (melo)dramatic, and beautiful-but-with-tinges-of-melancholy. Speaking of the latter, for my money, 'All I Need' is one of the best unrequited love songs ever.
Beyond all of this, though, In Rainbows is Radiohead's most listenable and downright fun album yet. Yorke's voice is at the heart of every song and carries the emotional heavy lifting while the rest of the band deliver on every level. The rock songs rock, the ballads tear at the heart strings, and the other stuff does whatever and sounds cool. I will admit right now that this may not be my favorite Radiohead album, but it's their most accessible and I-wanna-listen-to-it-over-and-over album. I simply can never get enough of it, its melodies, hooks, and sounds burrowing deep into the pop center of my music brain. I mean, I've had it for over a year and it still frequently floats around in my car for opportunistic listening. I chalk most of this up to how great the songs are. Radiohead were always an "ideas" and "sounds" sort of band, but who knew they were a "songs" sort of band, too?? I guess I always have but didn't realize it until now. Witness the clanging percussion and haunting chorus of voices on 'Reckoner', which produces an indescribable atmosphere that I'll never forget as long as I live. Witness 'Nude', finally completed after kicking around since the OK Computer era, and to these ears at least better than all of the previous versions I've heard. Witness the absolutely stunning 'Videotape', which is arguably the best closing song from a band with no lack of great closing songs. That it is the goodbye message of a dying man, ending with the line "today has been the most perfect day I have ever seen", gives me goosebumps every time I listen to it. Pay attention to the way the drum loop slowly collapses in on itself over the course of the song...it's one of those perfect touches that you may not notice right away because hey, this is Radiohead's pop album and you don't have to listen close...but give this album another listen and those little details and sounds you've come to love Radiohead for will be there.
Certainly, touring these songs before finishing them, playing at American music festivals like the hippie-ish/classic rock-y Bonnaroo, and being free of their record contract contributed to the sound of In Rainbows. But I don't need to talk about those elements. Of all their albums, this is the one Radiohead release that doesn't need any kind of context or explanation to enjoy. It may not be their best album in my estimation, but we should be so lucky that other bands would release pop albums of such accessibility and enjoyment while still retaining an edge and depth.