Frustrated Despair and Doubt
One of Tennyson's trademarks is his frustration at being unable to express himself the way he wants to. Wait just a second—the man was the Poet Laureate of England, so he must have been able to express himself...right?
Well, sure, but frustration is a common stance that the speakers of his poems take (you might also want to check out "Break, Break, Break" for another example of how this works). In In Memoriam, this comes through in how the speaker is frustrated by his own despair and doubt. He admits that some things he sees he "leave[s] unsaid" (1390) because words are puny things when it comes to some of the huge concepts he's tackling.
We're sure you've heard the phrase "Nature red in tooth and claw." Pretty famous, huh? Well, what about "'Tis better to have loved and lost than never to have loved at all"?
Before you read In Memoriam, you might have been tempted to think that someone like Willy Shakespeare coined these phrases. (Really: if you're ever in doubt as to where a pithy quotation comes from, Shakespeare should be your go-to choice.) But in this case the quotes are all 100% Tenny. He's the man.
Many of his other poems also feature quotable quotes, like "Ulysses" ("To find, to seek, to find, and not to yield"), and "The Charge of the Light Brigade" ("Theirs is not to reason why...").
Mythology Meets the Mundane
Tennyson is also a master at combining high-falutin' mythological concepts with everyday occurrences. Here, we get it in several places when the speaker imagines meeting the Muses, and Arthur going on his "Muses' walk."
He also manipulates mad mythology in "Ulysses" (particularly of the Greek variety) and "The Lady of Shalott" (where he gets his King Arthur on).
It's really not surprising why mythology might be on Tennyson's mind. Remember that Darwin's Theory of Evolution had just been introduced during Tennyson's time, so the issue of origins was very much on people's minds. Mythology gives a spiritual counterpoint to the more physical version out there in the scientific realm.