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Assuming you left December of 2014, that would mean the following has been added:
Rejuvinogi (which includes better inventory management, the update then removal of ticking time bombs, the addition of the "Dressing Room" which is a separate storage for repairable clothing that allows storage of 1 article each of unique clothes, instant bank transfer, search function for titles, etc.) Renovation [Combat] (which includes massive buffs to combat skills, including a number of new combat skills; some examples are bash which is like final hit but not, the new bleed/stun effects added to smash, and the ability to walk using counterattack with certain weapons) Renovation [Magic] (similar to the above, buffs magic; icebolt slows and recovers mana, firebolt does more damage, lighting bolt no longer expires, ice speared enemies can take damage but receive 50% unless used through an alchemy crystal) Renovation [Archery] (again, buffs? archery; adds urgent shot skill, seemingly messed with some speed related things but mostly buffs archery especially for atlatls) Renovation [Extra] (adds the dungeon for "Alban Training" and talents now award AP for hitting milestones) G20 (buffs crusader skills, adds a new crusader skill, adds new craftables) Dawn for Dungeons (Poulnabrone teleportation rock added...meh, hunting scroll quest rewards increased and made quest board for them near the new rock, dungeons were renewed but renewed dungeons are now maxed at 4 people, passes such as "Alby Adv for One" no longer exist and if variations exist those variations depend on party size alone, hardmode adv dungeons reworked and rabbie phantasm added, you may reenter dungeons if you relog within 5 minutes!!!) MusicQ (music was buffed, new music based dungeon in math, Renown System added (look for it in character menu) Momento (new storyline and dungeon in Abb Lake) Grandmaster Update (grandmasters buffed and you can now hold more than one at a time) Giants Update (giants were buffed hard, especially wind guard, taunt, and giant full swing; new endgame giant armor) Dan Update (made dan process solo and new skills added to dan, dan skills were visually updated) Flight Update (you can now fly in Dunby and Tir) Ninja Update (ninjas buffed) Chain Update (new skill set about chains, added missions for Baltane Unit) G21 (new raids and updated old raid system, new transform)
For more in-depth I would check out the wiki's patch page. Otherwise this will get you up to speed.