The Librarius Discipline doesn't have an overriding elemental theme, but is rather based on the collected knowledge of Astartes Librarians. In reality, that means that Librarius is a grab bag of updated Space Marine powers from previous editions.
The Emperor's Wrath is the Primarus power, and is a reasonably useful witchfire. It's an 18" range, S5 AP3 Assault 1 Blast, so it's threatening to most infantry targets. Nothing special but a solid basic power.
Veil of Time is a Warp Charge 2 power that allows the psyker and his unit to re-roll failed saves. This is an awesome power, and is really only limited by the fact that it only works on the psyker's unit. While this is useful on any Space Marine unit, it really comes into its own when cast on a unit with a 2+ save. It protects Terminators or Sanguinary Guard from the massed shots that normally wear them down, but it makes any units armed with storm shields really tough against everything. This is a great power.
Fury of the Ancients is a 20" beam power with S6 AP4 and pinning, and is an update of an old power limited to the Salamanders. While it won't do that much damage to most units, it is useful for forcing pinning checks on multiple units and is therefore strongest when paired with LD debuffs, such as those from the First Company Task Force.
Psychic Fortress is a blessing that grants all friendly units within 12" Fearless and Adamantium Will and a 4+ invulnerable save against witchfires. This is a very situational power, but provides defense against armies that rely on maledictions and witchfires. In most games, I expect you'll get the most use out of the Fearless bubble.
Might of Heroes is another old power that's come back. It grants the caster +2 to Strength, Toughness, Initiative, and Attacks. While there are other powers that grant +3 to each of these attributes (Iron Arm, Warp Speed), buffing all four attributes at once is quite useful. This is a great power for an aggressive Librarian (I can't wait to use it on Mephiston).
Psychic Scourge is a malediction that targets an enemy psyker. The caster rolls 2D6 and adds the highest to their mastery level while the target rolls a D6 and adds it to their mastery level. If the caster's result is higher, the target suffers a wound with no saves of any kind and can only harness Warp Charges on 6 for the next turn. If the caster wins by 3 or more, the target also loses a random psychic power. I'm a big fan of this power as it allows you to target and kill enemy psykers hiding within enemy units, and I enjoy nothing more than killing enemy psykers.
Null Zone is another blast from the past, and thank the Emperor that it's back. It's a Warp Charge 2 malediction that applies a -2 modifier to the invulnerable saves of a unit within 24". This is enough to render otherwise tough units that rely on invulnerable saves quite vulnerable. It's a great counter to Daemons using the Grimoire of True Names, Thunderwolf Cavalry or Bike Command Squads with storm shields, or Seer Councils.
Overall, I really like the Librarius Discipline. Part of this is nostalgia for the old powers that it includes, but it also includes counters to many parts of the game that I currently find troublesome. It provides you with unique tools for dealing with enemy psykers or Death Star units, both of which are currently prevalent. Veil of Time, Psychic Scourge, and Null Zone are all very strong powers, while Emperor's Wrath and Might of Heroes are both useful in most circumstances. Only Psychic Fortress and Fury of the Ancients could be considered weaker powers, but they're both quite helpful in certain circumstances.