As many people already know patch 3.1 is going to change twinking dramatically (if the current PTR changes go live). I have seen a few of the changes to 19 twinks mentioned in a few threads but have not seen a list of all the changes that I am aware of. Below is a list of all the current changes that are known to me and also some more useful information. I have also made a list of the leg items post 3.1 and a list of the librams and the enchants they can be used for. Lastly a few tips/fun things. After writing up most of this i've decided to make a guide for some of the less known stuff and post it when I have time.
1. BoA with Glyph of the Gladiator not giving stats.
2. BC-Leg Kits not giving stats
3. 100 HP to chest enchant requires character level 30 (150 HP can still be obtained see below)
4. AGM Trinket (Arena Grand Master) changed from +12 dodge rating to +12 stamina.
5. Naxxramas shoulder enchants no longer giving stats.
6. Magic Dust only lasts 10 seconds in PVP.
7. Gathering Professions above your intended level do not work. (Level 19 with max rank Lifeblood can not use the HoT, level 19 with appropriate rank herbalism can).
8. Most items made by Blacksmithing in the 1-300 skill range will be improved to be more useful.
9. New elixir - Elixir of Minor Accuracy (level 15 required): increases hit rating by 10 for 1 hour. Battle Elixir.
10. New enchant - Lesser Accuracy (feet): +5 hit rating
11. New cloak - Spidersilk Drape (level 19 required): 21 armor, 4 stamina 4 hit, 4 spell power.
List of the Post 3.1 Leg Slot Items for each class.
Hunters - Leggings of the Fang, Smelting Pants, Darkweave Breeches
Rogues - Leggings of the Fang
Druid - Darkweave Breeches, Sacred Burial Trousers (Horde), Scouting Trousers, Smelting Pants
Warrior - Chausses of Westfall (Alliance), Leggings of the Fang, Grunt's Legguards
Paladin - Chausses of Westfall (Alliance), Leggings of the Fang, Grunt's Legguards, Sacred Burial Trousers (Horde)
Shamans - Sacred Burial Trousers (Horde), Smelting Pants, Leggings of the Fang, Darkweave Breeches
Mage - Sacred Burial Trousers (Horde), Darkweave Breeches, Buccaneer's Pants, Pious Legwraps (Alliance)
Priest - Sacred Burial Trousers (Horde), Darkweave Breeches, Buccaneer's Pants, Pious Legwraps (Alliance)
Lock - Sacred Burial Trousers (Horde), Darkweave Breeches, Buccaneer's Pants, Pious Legwraps (Alliance)
Librams and there Enchants
Libram of Constitution - +100 HP to Head or Leg slot item.
Libram of Focus - +8 Spellpower to Head or Leg slot item.
Libram of Protection - +12 Dodge Rating to Head or Leg slot item.
Libram of Rapidity - +10 Haste Rating to Head or Leg slot item.
Libram of Resilience - +20 Fire Resistance to Head or Leg slot item.
Libram of Rumination - +150 Mana to Head or Leg slot item.
Libram of Tenacity - +125 Armor to Head or Leg slot item.
Libram of Voracity - Choice of +8 sta/spi/int/agi/str. to Head or Leg slot item.
Well Known/Little Known Tips & Tricks:
1. Pygmy Oil - Drink a lot of this and you will become a tiny gnome with a voodoo mask on.
2. 150 HP Enchant (or +6 stats) is still working on the Inferno Robes (Horde only) this item is obtainable at 19.
3. Formal Dangui can still get 150 HP or +6 Stats and you can also apply lesser and greater rune of wardings to this (has no effect in arena).
4. Frost Oil can be applied (through the trade window) to the Glacial Stone (Alliance) and probably the Runic Darkblade (Horde, not tested).
5. Dartol's Transformation Rod - You can get this during the Glacial Stone quest chain and stop doing the quest once you have the rod to be able to turn in to a furbolg (broken by damage).
6. Giant Feast (search on this site if you don't know how to use this as a twink) + Elixir of Giant Growth = Super Huge Twink.
7. Deviate Fish (not Savory Deviate Delight) can be very useful. Here is a list of the buffs/effects.
Rejuvenation Heals for 45 damage every 3 seconds for 12 seconds
Invigorate - Heal for 383-517 (can crit)
Healthy Spirit - Healthy Spirit 5 spi for 30 Minutes
Party Time - Cuases you to do random emotes.
Sleepy - Puts you to sleep for 30 seconds (unsure if it breaks on damage)
Shrink - Makes you smaller and appears to lower your Str and Sta.