I probably have around 50 or so quests, especially with all those miscellaneous quests you get going throughout the world. My only question would be, if I leave them all active would it effect anything outside of cursors on the map and in the game world? Like if one is 'active' too long could it fail? Or does it not harm anything at all to leave them 'active'.

Nope, doesn't hurt anything, just causes clutter.

Nothing active will fail. Having them active simply makes you map into a clusterF* of map-markers.

Doesn't affect anything at all. I don't know how they managed to pull it off, but everything works together extremely well, and you won't have to worry about screwing up one quest by doing another or waiting too long to finish it.

You know you can turn markers for quests on and off right?

Yes I know, but it's kind of pointless to first make all quests your active quests and then turn off map markers when you could just leave everything unactivated.

Thank you everyone for the help!

You can fail certain quests (choosing a side in the war), by advancing a competing quest. Most of the time, it won't matter how many you've accepted/what stage of completion they're in though.