I'm sure others can think of many recurring motifs Tolkien's work, but the ones that especially stand out for me are:
I don't even know how many orphans there are in Tolkien, but there are a lot. Frodo was orphaned at an early age and eventually adopted by his uncle Bilbo. Aragorn lost his father early on, then his mother and was raised by Elrond. Elrond himself was orphaned as a small child when Elwing and Eärendil sailed off to Eldamar and ended up saving Middle Earth but never came back. Eowyn and Eomer are orphaned and raised by their uncle. Faramir and Boromir lose their mother at a young age. In The Silmarillion there are many, many more examples. The fact that Tolkien himself lost his parents, his father died when he was quite small and his mother when he was 12, no doubt plays a big role here.
Man of the people marrying up
This happens frequently in Tolkien's stories. If the two parties getting married are not of "equal" standing, it always seems to be the man who is of "lower" birth than the woman. This is further emphasized when it is elven women marrying human men: elves being the innately more noble and refined race (not to mention immortal) compared to human beings. Aragorn and Arwen, Beren and Luthien, Idril Celebrindal and Tuor... Even in the case of Galadriel and Celeborn, it can be argued that she is "above" him since she was born in Eldamar while he had never been to the lands of the farthest west. Does this mean that Tolkien had an idealized and romanticized view of women? To some extent probably yes, though imo he shows few signs of misogyny in his work and has actually created some of the most interesting and complex female characters in speculative fiction. I'll write more about that another day!
The reluctant father-figure
This kind of goes with the above theme. The father-figure who is reluctant to let the couple in love get married is a recurring plot point in Tolkien's work. This very much mirrors the situation Tolkien found himself in when he wanted to marry Edith Mary Bratt (the ur-Luthien and! another orphan!) but got the cold shoulder from his guardian. To quote Wikipedia:
His guardian, Father Francis Morgan, viewing Edith as a distraction from Tolkien's school work and horrified that his young charge was seriously involved with a Protestant girl, prohibited him from meeting, talking, or even corresponding with her until he was twenty-one. He obeyed this prohibition to the letter, with one notable early exception which made Father Morgan threaten to cut short his University career if he did not stop.This same situation crops up with Aragorn, Elrond and Arwen. It's very significant for the tale of Beren and Luthien's relationship that her father Thingol is against their union. True love usually perseveres, even against horrific odds and it's not just the man who has to show his bravery and perseverance: in both the Aragorn/Arwen situation and especially in the Luthien/Beren story, the woman shows great courage and tenacity too.
On the evening of his twenty-first birthday, Tolkien wrote to Edith a declaration of his love and asked her to marry him. Edith replied saying that she had already agreed to marry another man, but that she had done so because she had believed Tolkien had forgotten her. The two met up and beneath a railway viaduct renewed their love; Edith returned her engagement ring and announced that she was marrying Tolkien instead.
Jealousy but no cheating
I can't think of a single occasion when either a married man or a married woman cheats on their significant other in a Tolkien story. Not even evil men do this it seems. Maybe it's because Tolkien thought cheating on a spouse too base to contemplate for his style of storytelling, but I still think the omission is interesting. There is plenty of jealousy. Maeglin the elf is jealous of Idril and Tuor's love for each other. Daeron the minstrel is jealous of Luthien and Beren. There is lots of jealousy and greed in Tolkien's stories, but again, no unfaithfulness. Maybe Tolkien never experienced it first hand? I can only speculate, but I think that on the whole, he must have been a rather happily married man.
Children are good
I also think Tolkien must have liked children. (And I don't say this just because he wrote children's books! Lewis Carroll did that too, but I don't think he really liked children very much at all.) Children are not featured all that much in LOTR or The Hobbit, but when they're seen, they are usually fun-loving, playful, endearing and rather intelligent. Look at Bergil, Beregond's son who befriends Pippin in Minas Tirith. Read the description of hobbit children in LOTR when they're mentioned in the description of Bilbo's party. Remember Farmer Maggot's household and his children. Children in Tolkien's world seem to be essentially good, though they are sometimes hit hard by tragic circumstances, but they are never annoying, stupid or "in the way". The only time someone talks disdainfully of children, it's in the words of Saruman, speaking to Theoden: "What is The House Of Eorl but a thatched barn where brigands drink in the reek, and their brats roll around on the floor with their dogs!"
I find this affinity for and often clearly expressed liking of children one of Tolkien's most endearing qualities. There are other recurring motifs of course: spiders, darkness, trees... but this will do for now.
Images of Frodo & Sam, Arwen, Elrond and Faramir & Boromir from the LOTR movies, thanks to TORN and Tolkien Gateway.