I think bugs/clitches are sadly a fact of life when you look at such games. For a number of reasons:
1 - the number of potential scenarios a player may invoke can be very high - from what they do, to what items them might have - the more scenarios the harder to test the more likely for bugs to creep through - could be something like if you happen to have finished quests A, B and D but not C and have 4 items of Z in your inventory you trip a bug that wouldn't surface in other scenarions
2 - the open nature makes AI and navigation a nightmare - hence all those characters walking into trees or whatever
3 - the size of the area makes it hard to check every bit of scenery is perfect, there are no 'holes' to fall through in the world, etc.
4 - they are expensive to make and QA leading to more chance of 'ship now, patch later' mentality with developers
Mostly though I think it's the difficulty of checking the code works fine with every permutation possible. Most games limit you to where you might be and what you might have plus what AI needs to cope with at any given time whereas open world titles really have to handle what feels almost like a magnitude of extra difficulty in this regard.
Look on the bright side - often the bugs and glitches give rise to some pretty funny stuff.
Try to be reasonable... its easier than you think...
