what has changed?

i havent played poe since 2013, and would like to know if many things have changed.
would it be good to continue with my old character?
or would it be better to create new one to be able to experience new content(if there is any)?

Start a new character in the upcoming 1-month league :)

http://www.pathofexile.com/forum/view-thread/1218688

They've had two new expansions since the beginning of 2014.

One expansion introduced corrupted zones and Queen Atziri as an optional boss.

Another expansion, introduced Masters and a hideout that you can decorate. You can complete master quests to level them up over time and add a few stats to items with their crafting tools.

strongboxes are something new you'll see a lot of. they are basically loot pinatas that can 1) kill you and 2) you can "improve" by spending currency on.

The game has changed massively and it's not worth resuming your old character in standard.

You should play the one month league(starting in a couple days). Learn the new content that's been added while you were gone so you can be prepared for act 4 when it hits.

Here's a partial list:
* Strongboxes (think trap chests that can be rolled magic or rare for extra drops and harder guards).
* Atziri (optional end game boss with fixed unique drops).
* Corrupted zones (small optional side areas with a tough boss that give you a chance at getting the items needed to unlock atziri).
* Vaal gems(super powerful skill gems require you to kill a certain number of enemies to activate).
* Masters (npcs that provide extra quests and allow you to craft mid-tier mods onto items at a fixed currency cost).
* Tormented spirits(like treasure goblins but they can possess rare enemies and touch whites/blues to make them more powerful).
* Lots of new skill gems.
* New map mods.
* Many new uniques(check the list on the wiki, there are so many now that they don't all fit on a single page anymore).
* Map system got reworked so you don't have to roll maze/area % every map(I think this actually happened at the end of 2013 but not sure).
* Passive tree massively overhauled.
* Many skill gems buffed and nerfed(lots of balance changes between this and the passive tree overhaul).
* Nemesis mods for rare enemies, bloodlines mods for blues (these can add extra difficulty or loot to the encounter).
* Guest monsters in higher level zones (basically you can see enemy types from other areas in your favorite farming spots in cruel/merciless).

A lot.

There are a lot more potentially lethal mobs / mods that can kill your character quickly and/or frustrate you if you aren't aware of how to deal with them.

Examples include ...

1. Nemesis mods such as corrupting blood and soul eater
2. Bloodline mods such as phylacteral link, bond of chaos, and shared boon
3. Tormented spirits can be very dangerous even if you know what they can do, when paired up with the "wrong" rare or unique.
4. The "ice nova" affix on strongboxes can be instantly lethal
5. Beyond unique bosses (and invasion bosses) can catch your character by surprise with how hard they hit.

I think there may also be a new exile or three who can be very dangerous.

Also, be on the lookout for new bosses added to some areas and for changes to some of the original boss fights (ie: Merveil).

Welcome back and good luck!