C – core set
M – Shadows of Mirkwood cycle
M1 – Hunt for Gollum
M2 – Conflict at the Carrock
M3 – Journey to Rhosgobel
M4 – Hills of Emyn Muil
M5 – Dead Marshes
M6 – Return to Mirkwood
K – Khazad-dûm deluxe
D – Dwarrodelf cycle
D1 – Redhorn Gate
D2 – Road to Rivendell
D3 – Watcher in the Water
D4 – Long Dark
D5 – Foundations of Stone
D6 – Shadow and Flame
H – The Hobbit saga
H1 – Over Hill and Under Hill
H2 – On the Doorstep
N – Heirs of Númenor deluxe
S – Against the Shadow cycle
S1 – Steward’s Fear
S2 – Drúadan Forest
S3 – Encounter at Amon Dîn
S4 – Assault on Osgiliath
S5 – Blood of Gondor
S6 – Morgul Vale
Much more often than not Boromir (M5) will only manage to raise your threat higher than to your liking; however, there are rare instances when this can be beneficial (coop, Osgiliath, Pelargir, for instance).
Dwalin (K) is situational; there just aren’t always Orcs around (and, shush, he’s not really the film version when it comes to fighting).
Nori (H1) is much better at keeping the threat low than Dwalin. Having many Dwarves in a deck is a ground to many a Dwarven strategy, and Nori’s ability benefits from that.
Glorfindel (D5) needs a mention as he’s the first hero making three-hero secrecy line-up available. And as for Boromir above, his (dis-)ability can even prove useful sometimes.
Wandering Took (C) gets a very abusive combo mentioned below; on his own, he can still be useful – not just for his ranged and sentinel potential.
Damrod (N) seems unimpressive in his ability, certainly needs building around if not played for his stats (which are just alright for the cost). As Spirit excels at keeping threat low, and enemies in the staging area, those two things can be further supported with Damrod leaving play after he’s done his duty.
Are there many cards that read Doomed X? Yes? Then Elfhelm (M5) is the man for the job, but do not expect he’ll help your friends across the table.
Ever My Heart Rises (D4) is great when Mountains and Undergrounds are around, otherwise it is useless; not a good card design unless you only build decks knowing what quest you’re up against.
Song of Eärendil (D2) seems made for Strider (see below) or the Wandering Eärendil combo; it is useful a plenty in other combos, though, especially as you automatically draw a card and you can fetch it with the Minstrel.
If you’ve got some unique Noldor in your deck, you might as well consider Elrond’s Counsel (D3). If one of your heroes is Noldor, you’d (probably) be crazy not to have this card three times in a deck.
If a player has many attachments to be attached on other players’ cards, the others should consider adding Renewed Friendship (D1). The threat reduction is only a part of it. Coop only, of course.
Smoke Rings (L1) require Pipes to be effective, so more Pipes there are, the more reasons to run this card.
The Galadhrim’s Greeting (C) is the ultimate threat reduction card, though Gandalf is never far behind even if the Galadhrim’s are ahead; this card is especially nice to help other players.
In his Strider version, Aragorn (D3) can really save the day by bringing the threat level down to its initial level. This opens up several strategies which notably don’t have to include Spirit for threat reduction.
Mirlonde (S2) is certainly not very impressive on her own (despite the attractive looks). She seems fit for mono-Lore decks where she'll do as much as lower the initial threat of the hero line-up by 3. Well, one can still hope for some great Silvan synergies in the future.
Needful to Know (D1) is one of those lesser threat reducing cards that has an alternate use, being a secrecy card, though, makes it harder to find a good deck for.
Gandalf (C) can do many things, reducing your threat is among them.