I prefer situations like Virmire and the Suicide Mission where you have some sort of control. Either we're forced to make a decision between two people (or maybe in this case, two planets?), or a character's survival depends on previous actions. Those are preferable, mostly because it forces someone to live with the consequences of their decisions. I won't go so far as to say that every death must or should be avoidable, but I like having some say in the matter. The amount of people I can't save in Dragon Age II, for example, is a fine example of why I won't replay the game. I don't want to put myself through all of that inevitability again.
The next part of my response will contain SPOILERS.
A death with closure helps a lot. Since Wash was mentioned, I preferred Shepherd Book's final moments in Serenity. Allowing him to get something out in that moment of dying meant something. I kept watching the film uninterrupted. However, when Wash died, I had to pause it. I never saw that coming, and I kind of had to take a moment and go over what just happened.
I'm going to hate myself for going further, actually. I prefer something like the Shepherd. Still, Wash might have been far more motivational, had Serenity been a game. That sudden, unexpectedness of his death could be a huge driving factor behind why you have to keep fighting. It's like when Garrus drops down during his recruitment mission after the gunship comes into play. Those deaths can be a catalyst to keep going. Thankfully, Garrus didn't die there, but the thought that he might have inspires fighting harder (and once your enemy is dead, seeing if your friend is alive or dead).
I think that I only have one request regarding deaths. I don't want someone who was romanced to just die. If I romance Miranda, for example, I don't want her being shot down and that's the end of it. For closure's sake, let love interests get a goodbye. Even if it's Miranda over a comm link telling Shepard goodbye because she knows that she won't come out of whatever mission she's doing, that's better than nothing. I think romanced characters deserve a goodbye.
Also, I hope that companions or love interests that die get closure in another way. If female Shepard and Kaidan are on the battlefield and Kaidan gets killed, I understand that she can't just stop fighting. It happens a lot where a companion, friend, etc. gets killed in the journey, but there's no real grief or mourning. Coming back to Serenity, the scene at the end with the group standing around the graves of those who were killed was powerful to me as well, especially Zoe's part in it.
So, to sum that up:
Please, no "Wash" deaths for love interests. Some kind of goodbye would be nice. Although I'm not against them. They're certainly striking in their own right.
Virmire "this or that" decisions are great, and so is the Suicide Mission. You have control, but in the former you have to live with the fact that you can't save everyone, and in the latter you learn that the decisions that you make matter.
I like goodbyes. I've never seen Wrath of Khan so I can't comment directly on that, but I like having that opportunity. Here's a scenario on how the Suicide Mission could have played out that I would have probably cried over but I would recognize as an acceptable death: imagine that the loyal tech expert climbs through the vents and successfully gets Shepard and company through the doors... But they're trapped in the overheating vents. Over the comm, they get to say goodbye, but they did succeed in their task. That situation would have been heartbreaking because I would have known that I chose that person for the task, and while they completed it and were the best (or one of the best), they still died from it. Picking the right person for the job doesn't always have to mean that succeeding doesn't come with a price.
Another example of character death? The very first one that I experienced: Aerith. Oh, how I cried. That's another example of the effects of an unexpected death. But those moments, like I said above, can really give you the drive to keep going.
As I began with, though, I prefer saving people. I hope that the deaths will be used sparingly. I expect people to die, even those close to Shepard. Still, unless we've just made all of the wrong decisions along the journey, that the deaths and sacrifices of people we've come to care about aren't dramatically high. If we made wrong decisions, and characters close to us have to sacrifice themselves to make up for that, then that's also an acceptable approach.
I'll make an example of what I mean by that, and then I'll stop rambling.
Say we didn't complete the UNC mineral collection quest. Let's say in Mass Effect 3, we're on a besieged colony that never received certain supplies because there weren't enough resources to go around. This colony now can't sustain itself in an attack from the Reapers, so Ashley takes a nearby warship of some kind and does a suicide run with it into a Reaper vessel.
That's not a perfectly thought-out example, but that's what I mean in terms of a companion or person we care about picking up the slack and sacrificing themselves because of something that we, the player, as Shepard, either failed to do or accomplish, or made the wrong decision. I think that it really would be a powerful statement if something such as the mineral collection, a quest that could seem rather out of the way or insignificant, really had an impact in such a great way.
Alright, I think that's as much as I can organize from thought to response right now.