Bungie released the first info about the new Weapon Balance patch to be out for The Taken King, the infamous 2.0.0 patch.
All weapons will have some kind of changes and some loved weapons received some unpopular nerfs that ended making even non-gaming websites/magazines talks about this big news.

Below I tried to sumarise all the changes but it will always better to read the original news:

https://www.bungie.net/en/News/News?aid=13147

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AUTO RIFLES

• Increase base damage
• Start damage falloff closer to the player to emphasize its role as a close to medium range weapon
• Small reduction in base stability. Landing shots at optimal range is unaffected, but repeated precision hits require more weapon control to land consistently
• Boost damage by 10% against AI combatants

PULSE RIFLES

• Reduce base damage of the medium RoF by about 2.5%
  • PVP: Bursts-to-kill (all precision hits) is 2 or 3, depending on victim’s Armor stat
• Small reduction in base stability. A burst should still land all shots at optimal range, but three precision hits will require more weapon control to land consistently
• Increase magazine size on for all base inventory stats

SCOUT RIFLES

• Increase base damage for medium to high Rate of Fire Scout Rifles
  • PVP: Does not affect TTK in PVP on a Guardian with full health
• Increase magazine size for all base inventory stats
• Reduce Final Accuracy when firing from hip
  • Fast firing outside of ADS will be less accurate
• Boost damage by 5% against AI combatants

HAND CANNONS

• Start damage falloff closer to the player to limit long range lethality
• Small reduction in ADS accuracy, targeted at making long range snap-shooting less reliable
• Reduce final accuracy when firing from hip
  • Fast firing from hip is less reliable
• Reduce magazine size for all base inventory stats
• Reduce base Optics (zoom) for all Hand Cannons
  • ADS now grants more width in favor of depth

SHOTGUNS

• Shotgun perks that enhance lethality at range should be less effective when combined with a high initial Range stat:
  • Reduce Shot Package Accuracy buff by 30%
  • Rangefinder adds a 5% base Range increase on ADS (was 20%)
• Reduce Precision Damage multiplier on Shotguns by 10%
• Reduce damage against AI combatants by 10%

FUSION RIFLES

• Slow charging, high Impact Fusion Rifles have decreased Range values
  • Makes it more difficult to max out Range for these weapons
• Projectile Speed for Fusion Rifles is slightly reduced
  • Emphasizes the need to lead a targets outside medium range
• Improve Accuracy for short range Fusion Rifles
• Reduce Accuracy for long range Fusion Rifles

SNIPER RIFLES

•  Final Round on Sniper Rifles buffs precision damage only, not base damage
  • This change only affects this perk when combined with Sniper Rifles

ROCKET LAUNCHERS

• Slight increase in base Blast Radius.
• Grenades and Horseshoes proximity detonation reduced.

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Hard Light

• Increase base Stability to 80 (was 65)
• Increase bounce count for Hard Light projectiles
• Hard Light projectiles are not affected by Damage Falloff

Necrochasm

• Increase base Stability to 60 (was 40)
• Increase magazine size
• Cursebringer perk will always trigger on a precision kill
• Cursebringer explosion has increased radius and deals more damage 

The Last Word

• Reduce Range stat to 10 (was 20), reduce Stability to 20 (was 30).
• Reduce effective range while in ADS.
• Increase accuracy and precision damage aim assist scale when firing from the hip. “Get three coffins ready.”
• Fixed bugs with Hipfire damage bonus applying incorrectly 

Thorn

• Reduce base damage of Thorn’s Mark of the Devourer DoT (Damage over Time) to roughly 1/3 of what it was in PVP and PVE
• Allow DoT to stack up to 5x across multiple landed projectiles
  • This is a net buff for Thorn’s DoT, but reduces the lethality of the weapon in PVP 

Hawkmoon

• Add a stack limit to Luck in the Chamber and Holding Aces so that only 2 of the bonus perks will ever stack on one round. This should prevent Hawkmoon from 1-hit killing full health players in PVP
• Add 2 rounds to Hawkmoon’s magazine when Holding Aces is unlocked
• Luck in the Chamber damage bonus reduced by 3%

Ice Breaker

• Increase recharge time for Ice Breaker rounds to 1 every 8 seconds. Was 1 round every 5 seconds

No Land Beyond

• Increase weapon handling speed for faster time to aim, ready, and stow
• Adjust sights to fix overlap/parallax issue while aimed
• Increase time decay of The Master to 8 seconds
• Add an additional 20% Precision Damage bonus while The Master is active

Black Hammer

• Increase ammo inventory to 18 rounds
• White Nail perk now pulls ammo from your inventory

Lord of Wolves

• Tripled recovery boost bonus for allies granted by Lord of Wolves 

Gjallarhorn

• Reduce damage of Wolfpack Rounds

Some weapon stats on Crucible.

never owned the g-horn(i've tried to get it)

the Black Hammer nerf isn't as bad as people are complaining; it was ridiculously OP when it came to any boss fight(specially a solar burn nightfall)

the icebreaker nerf is stupid.....now we have to wait more for our bullets to replenish....I still don't get this nerf, no one was abusing the already 5 sec. waiting time

auto rifle getting a buff is welcomed

hand cannons getting nerfed was not....the reason why hand cannons were OP(impact/ability wise) was because of their slow rate of fire(excluding the TLW) when it came to the other primaries; the thorn was the only HC that needed nerfing as the corrosion stacking was BS imo

Yeap I agree.

The Shotgun range nerf is good too.

About Thorn seems like you won't get kill by two headshot + DoT now... that makes it fair.