I find that "open-space" is often confused for "open-world".
Just because something is a wide-open space, doesn't mean it's a populated world.
A true open-world game has other characters besides you, doing things that you don't necessarily have any control over.
That's one thing that DA:I had spot-on regarding your companions; outside of direct combat, you did not have any absolute control over who they were, what they liked, and what they do.
Your interactions with them could exert an influence, and it was gradual.
First they think you're not so bad, then they like you, and eventually they will stand with you 'till the bitter end.
Or they think you're a bit of a jerk, then a real creep, and eventually abandoning you to your fate.
With Mass Effect, it's relatively simple to get everyone to like you (more-or-less) and be a HERO; or get everyone to fear you and be an ANTI-HERO.
I enjoyed it, and it works well for the series.
WIth Dragon Age, not everyone is going to like you because several character interactions are "exclusive", i.e. you say one thing and Companion A "Greatly Approves" whereas Companion B "Disapproves".
The complication is interesting and thought-provoking, leading to role-playing mind-games like "Why would my character say this?" or "Why would he feel this way about this circumstance?"
Whether it's the binary simplicity of Mass Effect or the branching spiral of Dragon Age, the dialogue needs to take on different flavors based on what option you pick, but isn't completely at your beck and call. Influence, not necessary absolute control.
-------------
One thing I like about Skyrim was that things happen that you have no control over:
You leave Breezehome and suddenly a vampire and his thralls start attacking the townspeople. Here you are ready to go the College of Winterhold to learn a new Restoration spell but now you have to break out your weapons and combat magic to quell the threat. All the while hoping you don't incur an assault charge by accidentally hitting the guards.
And then the vampire kills one of the citizens, who had given you a quest previously, except now that they're dead, you failed the quest.
Something like the above, where you have no idea it's happening, no way to prepare, you just have to grit your teeth and get through it.
It doesn't have to be earth-shattering, or "epic", or anything like that, but it gives the world LIFE, it shows REAL, LIVING people and events that are happening whether you do anything about it or not.
It shows that there are other powers in the world other than yourself that have wills of their own.
A good open-world scenario does that. It gives the player just enough power to get started, but also gives other entities in the world power as well.
To be clear, I'm not suggesting ME:A needs to be a "SKYRIM IN SPACE", but I feel BioWare could take a page out of Bethesda's book regarding how to take an open space and make it an open world.
Put simply, have NPC's act on their own without needing the protagonist to "activate" them (scripted sequences are obviously exempt, for story reasons), and have some events that are "semi-scripted", i.e. it makes sense in context, but it doesn't announce itself with flashing lights and sirens, and it puts you on the spot.
It's not a massive endeavor, but it goes a long way to making your open space an open world.