So much has changed since you stopped it's impossible to go through it all. Your best bet would be to play around a bit and come back with specific questions rather than something so general
I'll have a stab at some brief highlights
1) Dark Below and House of Wolves DLC missions now integrated into story
2) Character level is now no longer attached to your gear and you raise it by earning XP only. New level cap is 40.
3) You now have a new stat called light level, which shows how powerful a guardian is based upon their gear. Gain better gear to get better light level and take on more difficult enemies
4) Crucible marks & vanguard marks have been abolished and combined into legendary marks
5) Class specific armor materials have been abandonned and now all calsses need a single "Armor Material" to rank up their armor or "weapon parts" to rank up their weapons.
6) Stat values have been normalized across all items to make them directly comparable. 170 is the max defense value for any piece of armor and the max attack value for any year 1 item. All year 2 items appear to max out at approx 280 defense / attack.
8) your light level is now calculated based upon the defense level of your ghost, and your class item, and at rank 40 a new item slot called artefact
9) you can now swap out ghosts to gain more defense and gain new perks
10) Much more emphasis on quests and less emphasis on repattive missions
11) Lots of existing content has been "questified"
12) voice of ghost has been replaced by one voiced by Nolan North
13) You can now upgrade weapons using legendary marks and they break down into legendary marks, simplfying the number of in game currencies.
14) nightfall no longer gives a weekly XP buff
15) weekly heroic strike that gave strange coins has gone
16) strike playlist has been overhauled and simplified. Now grant strike specific rewards and number of difficulty levels available reduced.
17) taken king adds a new patrol destination - the dreadnaught.
18) bounties can now be handed in from quest screen, progress can be tracked through your ghost and you can hold 16 of them plus quests
19) 5000 XP bounties now gone, replaced with easier 3000 XP bounties, to be supplemented through additional much harder weekly bounties with bigger XP.
plus lots of new content. Those are the main stand out ones for me.