What makes you play Open World RPGs for 50+ hours?

considering you have 1,395 hours in TF2 I'm going to just assume you're more of a gameplay and online deathmatch kinda guy/gal.

But if you try again you owe it to yourself to try with Fallout: New Vegas.

Fallout 3, New Vegas, Elder Scrolls 3-5 are great examples for open world games. And what keeps you in the game, playing? When you go to do a quest for example, the Mage's guild. Then your way you see a deer and jokingly try to hunt it. Then accidentally go to random cave with nexromancers. Kill them, find some nice loot, maybe some loot / notes - story about the cave, even a quest. On your way out from the cave you see a strage stone only in 3-4 minutes of running from there. You start going there. Aaand you find even more adventure, and probably you'll forgot about the quest you started to do if you're not paying attention :)
TLDR: always something intresting/ something expensive/ (:3) something killable/ something from you need to flee / something adventure lurking at every corner. In falllout 3 I started going from A to B, then ended up discovering a dozen locations near the two, even when the route would take only ~5 mintues normally :)

cuz it's funny

Because my internet is down so I have less distractions. Its how I finished Dragon Age: Origins.

because of that "what comes next, what will I find, what will my next armor look like" feeling

Generation that watchs short youtube sketches with kittens and never read any book except those in school cant focus enough to stay long with a good story.

Because I'm complicationist, or at least to some degree, so side quests and world exploration keep me doing staff out of main story.

And Just Cause... too, its just fun to fly in speed boat, etc.

Random events, encounters. Hidden side quests or collectables.

What keeps me playing. Apart from many places to discover and sidequests to do? One word - MODS(almost 500 hours in Skyrim).

The question answers itself. The games are openworld.

This thread is making me play fallout again...

Fun.

Two Words: Completionist OCD :D

Rich lore and intresting sidequests / side-chains storylines. If all RPG offers for side quests is go here, kill the bad guy and that's it, go look for other quest giver I will just stick to main storyline. But in some games (TES series is a good example here) sidequests storylines can even be more epic than main story :>

I've put hundreds of hours each into Morrowind, Oblivion, and Skyrim...

Without finishing the main story in any of them.

One word, FREEDOM!

There are so many: characters to talk to, stories to hear, quests and sidequests to complete, monsters to kill, weapons and armors to collect, vanity items to obtain, books/notes/messages to read, easter eggs to find, game mechanics to learn and master, skills and spells to try, puzzles and mysteries to solve, ways to approach a specific quest, races and classes to play, factions to join; and I could go on and on.
I've done and still do all this in two absolutely amazing open world, free roaming RPG series: Gothic and The Elder Scrolls.

Freedom :)

Well, just all the others things there are, like, when I initially started Skyrim I spent ages doing side quests, exploring, leveling up skills etc. I didn't even finish the main quest until maybe a year afterwards. I didn't play the game with the intention of beating it, putting it aside and playing a different game. I just played it, and enjoyed almost every minute of it, doing whatever.