Big thing these days, and for some of us it has been a big thing for more than decade.
They are expensive to make, usually prone to extreme buggyness that ideally requires a QA phase well beyond the resources of any game development company both what regards time and amount of testers - all that translates to money as well.
And, they are very fun, when they don't hopelessly break.
I have nothing against this 'trend', and welcome the latest news of Star Wars game being developed in an open-world setting. Witcher 3 is likely the second game I've looked forward to with such anticipation (DX2 being the first).
In this thread we can perhaps think when some games got it 'right', and when they didn't.
My best memories are with Morrowind (TES3), but I have given Fallout:NV and Fallout 3 both two runs, separated by few years. Coming back to these games years later allows one to experience the world from a fresher perspective, and enjoy it once more.
Since FO3 is precisely the game I happen to be playing right now, it is interesting to compare it to :NV - which I played not long before - as far as open world mechanics go. I am several years past the 'main quest disappointment' phase that led to uninstalling the game, and somehow the grey concrete jungle does not bother as much either.
I can immediately notice something that :NV failed miserably at, at least in my opinion: it wasn't true open world or at least was not designed as such. In :NV they created a massive wall of Deathclaws and those giant hornet things that as a low level you couldn't easily penetrate, forcing you to go on a preset route around the map rather than across it. No, I did not let that bother me and actually broke through the hornet corridor at fairly low level, but all elements considered I realized that they expected me to progress towards east, in the direction of the highway.
This is something you don't do in open world game. Wall is a wall, whether live or a rock. Once I had gotten through that, there seemed to be a lack of.. continuity, perhaps? Sure, places to explore, but it seemed of much lesser importance as far as any quest or location content went. Hardly a reward for the efforts.
Another element in open world games is enemy distribution by strength (and type, but thats besides the point here). Bethesda games have preferred enemies that level (or are replaced) in correlation to your strength, and as much sense it may make it cripples the world immersion. It became a big joke in Oblivion already. I don't think :NV Deathclaw and Hornet placement was supposed to address this issue, rather than just forcing semi-linear gameplay on a tiny map.
Size of the maps - if you have open world game, you generally need a very large base map, or your game is just too tiny. I've touched into the matter of linear gameplay on supposedly open-world map, unimmersive enemy spawns, more challenging locations; map size is perhaps most important, and in turn will either fix or emphasise listed issues. With time the maps do tend to get larger, but as a mate of mine stated about FO3 (for example), there was location stuck in location and practically no wasteland as such. TES series in general were a bit better made in that regard, but only just.
Quests.. all the adventuring aside, your character should have some kind of purpose, should be able to leave some kind of impact in the game world. This does not immediately mean he needs to be a nth messiah, but pushing an epic main quest just to let it epically fail is not the right way to do it. Sure, Skyrim has established itself as a decent looking (for enough people) adventure world, and people don't mind its shitty MQ as much anymore. But lot of us are looking forward to a game that aims to do better in that regard. High hopes for W3.
Rant juice running out, contribute your 2 or 5 or preferred amount of cents on the topic of open world games past or present. Logically thinking, there is no reason why any game cannot succeed as an open world game (ie Deus Ex), it is just completely different matter how well developers can script and build it.