This:
You create a character. You can choose whatever you like for your character's features, within reasonable limits. For examplem you can choose a race and class, name, gender, appearance, equipment, etc. But the game is not expected to create a custom class just for you because you want to play a Steampunk Detective.
And:
But what if I want to play an engineer on a ship someone else (AI?) commands? The game can't do that, even though it's a legitimate expectation in the environment. Or maybe I want to play a drug dealer. Not a drug runner, which is just someone carrying illegal cargo. I mean going down to the colony and walking around on foot.
Don't seem to mesh well. You seem to be saying that if you are playing a P&P RPG you can be whatever you want within reason but if its a CRPG you should be able to be whatever you can imagine.
It's true that computer games don't offer the almost unlimited freedom of P&P games, but you can still RP in them. Sandbox games are great for this. I played Saints Row 2 with a friend over the xbox's system link, and while I couldn't run for mayor or summon eldritch horrors from beyond the stars, I still had fun RP'ing a semi-aware well-meaning psychopath.
Think of it this way, compared to most P&P RPGs mass effect doesn't offer you as many choices. You cant join the geth, or shove ashley out an airlock, or cut off your own leg just to see if the doctor would save you. But the within the context of what their pre-defined character would do, you have rather a lot of options. It's like turning up to play D&D and the DM has rolled all the PC's characters for them. You're still roleplaying, but you're doing it within the character they've created for you.
Okay, so my Steampunk Detective example was a little off. A better example is saying that in D&D you want to roleplay someone who runs an inn or trade guild or something. It's within the game's setting, it's a legitimate choice. But a computer game that tries to emulate D&D would probably leave that option unavailable, in the same way the space shooter / trader / explorer would leave out the option to play an engineer. Both are worthwhile and fun (if you like it) and make sense.
Of course you would not expect to be able to play a Jovian Particle Accelerator Engineer in D&D, nor would you expect to be able to play a Magic-User in a space sim.
I wholeheartedly object to saying that a game that gives you a role, and tells you to play it, is an RPG. This is why a D&D referee should never pregenerate characters. In a convention game, it's necessary, because of the circumstances of the convention (consistent scoring and difficulty, getting started playing right away) which are not present in a normal game (no scoring, choose your own difficulty through character choices, you can afford to spend a few minutes doing chargen). If you call FF7 an RPG, because it lets you play the role of Cloud Strife, then there is no reason not to call Bomberman an RPG because you are playing the role of a guy who has bombs. Or Pac-Man because you are playing the role of a ravenous ectophobic cheese pizza.
For this reason I call something like Saint's Row or Morrowind or Fallout 3 as something other than a Story Completion Game, but they're not Role Playing Games. Not quite. Maybe call them Sandbox Action-Adventure Games?
Here's an example of the possible player choices that I'd expect to see in an actual RPG:
Situation: A band of Goblins have seized a barn and driven the farmer and his family off their land. What do you do?
1: Offer to get rid of the Goblins for pay from the farmer so he can reclaim his land.
1A: Expect pay as a portion of his land rather than money.
1B: Expect pay in the form of the service of his son as a hireling.
1C: Expect pay in the form of his daughter.
1C1: As a wife.
1C1A: But actually don't marry her.
1C2: As a concubine.
1C3: As a party camp follower.
1C4: As a slave.
2: Leave. Pass by without doing anything.
2A: But spread word of the problem
2B: And spread word that the problem has already been solved
2C: and spread word that the farmer and family was turned into Goblins by someone who took his form and now lives in town
2D: After investigating and spying, but doing nothing
3: Drive out the Goblins regardless of the farmer.
3A: Because you're a nice guy.
3B: Because you're going to seize the farm for yourself.
3C: Because you're racist and hate the Goblins.
3D: Because you want information from the Goblins.
4: Drive out the Goblins by
4A: Burning down the barn
4B: Blasting the barn with non-fire spells
4C: Smoke them out
4C1: With green wood and leaves
4C2: With smoke spells
4C3: With illusion spells
4D: Sneaky assault
4D1: With assistance from illusion magic
4E: Frontal assault
4E1: With assistance from illusion magic
4F: Flooding them out
4F1: By diverting a stream
4F2: By collapsing the water tower
4F3: Water magic
4F4: Weather magic
4G: Trick the Goblins
4G1: By making them think the place is haunted
4G1A: Using nonmagical methods
4G1B: Using illusion magic
4H: Convince the Goblins to leave
4H1: By force
4H2: Through fast talking
4H3: Magical charm spells
4H4: Paying them
4H5: Doing them a service
4I: Rampaging herd of animals
4I1: Who were charmed magically
4I2: Who were summoned magically
4I3: Who were brought together by a wilderness-type character
5: Enlist the help of the local militia
5A1: And have them do all the work, and accept the danger
5A2: And split the work and danger
5A3: Do most of the work and keep them as a backup
5B1: By force
5B2: Through fast talking
5B3: Magical charm spells
5B4: Paying them
5B5: Doing them a service
6: Enlist the help of the local hedge wizard
6A: By force
6B: Through fast talking
6C: Magical charm spells
6D: Paying him a lot
6E: Doing him a service
7: Enlist the help of the gang of youths who think they know better than they do, and don't understand the risks
7A: By force
7B: Through fast talking
7C: Magical charm spells
7D: Paying them
And this is just what I came up with off the top of my head. It's an uncommon game that can let you do all that - but D&D will absolutely handle all of these with ZERO PROBLEM. That's why a tabletop RPG is so clearly different from a computer game.
Not to say we'll never see this happen. Computer tech is running ahead like crazy.