ANNOUNCER: And now, that brave youth who bore through snow and ice, the banner with the strange device:
COSTELLO: Heyyyyy Abbott!!!
The above quote (well, except for Costello's line) is from Excelsior. James Thurber may have contributed to the downfall of Longfellow when he included it in "Great Poems Illustrated," where he took over-familiar poems that kids were forced to memorize in school, and sent them up by drawing the characters as Thurber men and Thurber women. "Excelsior" was especially ripe for this kind of treatment because it's the kind of poem that presents itself as a serious work -- about pursuing your goals against all odds, a favorite Longfellow theme -- while the sing-song style and sheer pointlessness of it all (since the youth's goal is never defined beyond that meaningless "strange device," why should we care that he keeps carrying that damned banner?) make it seem more like bad doggerel; it has all the pretentiousness of high culture with all the silliness of low culture, a dangerous mix.
While most of Longfellow's poetry isn't as bad as "Excelsior," the things that once made him so popular are the things that ensure that he'll never have a high literary reputation again. Longfellow was a "public" poet; that's not to say that he never expressed his own feelings, but he was almost always trying to universalize those feelings, to say things that could strike a chord with his readers. And as a "public" writer, he was writing to be intelligible to the general reader, which meant fairly simple language and expression (which can make a poem seem uncomfortably close to Hallmark greeting-card verse). At his best, this style can be direct and powerful, as in the Longfellow poem that probably has the highest reputation today, "The Chamber Over the Gate":
He goes forth from the door
Who shall return no more.
With him our joy departs;
The light goes out in our hearts;
In the Chamber over the Gate
We sit disconsolate.
O Absalom, my son!
That 't is a common grief
Bringeth but slight relief;
Ours is the bitterest loss,
Ours is the heaviest cross;
And forever the cry will be
"Would God I had died for thee,
O Absalom, my son!"
But apart from the problems this direct, clear style creates in Longfellow's poetry -- like the morals that he tacks on to his ballads, telling us what the theme is and what we should take away from it all -- it makes Longfellow a loser when it comes to criticism. Literary criticism, as it has developed, values two things in varying degrees: personal expression, or decoding meanings. Walt Whitman, like Longfellow, tells us exactly what's on his mind, but he's expressing his own personal idyosyncratic feelings, not trying to be one with the feelings of his readers (plus, as he was kind enough to point out, he contradicts himself), so you can write an essay about him. Robert Browning left himself out of his poems, but he was Mr. Obscure, so you can write about him (plus, let's face it, Longfellow was no Robert Browning). With Longfellow, you know exactly what he's saying, but you don't know much about him, personally. Not much to write about there. And that's probably why I went through five years of university English Lit without studying a single Longfellow poem. I'm not saying Longfellow's once-exalted reputation was deserved, but his current un-exalted reputation isn't deserved either; it's based more on a bias against his kind of poetry -- direct, not in need of decoding.
The other thing about Longfellow is that in addition to being a public poet, he was sort of an "academic" poet, the kind of poet who experiments with obscure verse-forms and metres simply because he's familiar with them and wants to try them out, and who consciously tries to imitate the great poets he's studied. The famous "Paul Revere's Ride" is part of a longer poem called "Tales of a Wayside Inn," which is basically an attempt to do Chaucer's "The Canterbury Tales" except a) American and b) clean. Longfellow often paid tribute to or imitated his beloved Dante. And of course Longfellow was famous for using metres that had never, or hardly ever, been used in English poetry before; "The Song Of Hiawatha" is the most famous example, using the same plodding metre ("Should you ask me, whence these stories...") for a zillion lines, and all going to prove that there's a reason why this particular metre had never been used in English. You could say that Longfellow is a strange combination of a geeky professor and a celebrity. Come to think of it, these days there are quite a few geeky professors who are celebrities, or at least would like to be, so maybe he was ahead of his time, in that respect if in nothing else.
A good examination of Longfellow's strengths and weaknesses is this essay by William Long.