The thing about Colossus (aka Piotr Nikolievitch Rasputin) is that there's actually two things about Colossus, and they don't sit particularly well together (though one can draw a straight line between them). Which, as we'll see, is oddly appropriate.
Let's go with the most interesting part first. Some of you might not want to read this whole thing. Or read it at all. Or read any of these things at all, which just makes me a guy standing on a street corner waving a crumpled comic around. Which, let's be honest, is more interesting than a Big Issue seller, though any given Big Issue is somewhat less than 41,000 words long and doesn't insist on serialising itself over the course of two years. Nor is it likely to be this needlessly self-referential.
Let's go with the most interesting part first.
There's no such thing as a continuous variable. Not really. Not out there in the world, anyway, which is the only way a variable can be meaningful, and not just useful. The universe is a grid, the Planck values the spaces in-between each intersection. We don't walk through our lives, we jump from point to point, imperceptibly, solving Zeno's paradox in the simplest way possible.
If the universe isn't continuous, then, if the sliding scale is really only a ladder with rungs to close together to see, then that makes us a grid as well. Our anger doesn't swell, it increases by increments. God can measure precisely the degree to which we love someone, or hate them, or miss them, using the units only He knows.
Which is fine, in general, because for most of us it doesn't make any difference. For most of us, if you can't see the gaps, the gaps don't exist. We see a rainbow, not a collection of water droplets. The illusion is key. 
Sometimes, though, a person's gaps are all too obvious. Sometimes someone can only be described as digital, and the illusion breaks.
Colossus is digital. He's either flesh and blood, and just a man, albeit one of well-above average strength, or he's made of steel; an unstoppable powerhouse of cold metal. There is no in-between. Storm gets to play with the entire Beaufort Scale. Gambit can charge cards to explode, or just to light a room. Even Cyclops has a visor to stop him tearing off people's heads every time he does a double-take? Colossus either punches you through a wall, or he leaves you alone. There's just too many gaps between the two, and Colossus has to jump them all. Practice and experience might help around the edges, but there comes a point where an object is so heavy that it's either immobile, or it's going to crush you. Momentum is a scale as well, but it ticks up pretty fucking fast sometimes.
We've talked before about the perils of power. The more you have, the harder it is to use, because the more dominoes you'll knock over between here and there, where there is an unknown future where you're wife gets killed, or what have you. Wagner's Wotan couldn't apply his power because he knew applying pressure to any point would cause the universe to collapse around him. Black Bolt has the same problem, which if nothing else proves you can do far worse as a comic book writer than steal from German opera. You could even make an argument that Colossus' problems are a microcosm for the ungainly might of the USSR at the time of his creation, but since Colossus has never liquidated any peasants or tried to invade Afghanistan, I'm not sure how far you'd get with the analogy.
The illusion comes apart. Colossus isn't a superhero, he's a blunt instrument, or no instrument at all. He is a tool other superheroes use, when they can, and one they hope keeps out of trouble when they can't.
I guess it's no surprise Colossus is an artist. It's the one place the illusion holds. A sketch is a memory, not a collection of carbon molecules spread across paper.
Here's the other part. The lesser part. The totally shit part, if you will, though I guess your mileage may very, especially if you lie within the intersection of the Venn diagram of X-Men fans and people who think Mills and Boon novels are too much like hard work.
Let me know if this gets too tough for you. Colossus is strong, right? Really strong. So strong that he could be the biggest dick in the world if he wanted (c.f. Juggernaut). But he isn't. He's really nice. He is, not to get too technical, a "gentle giant."
Still with me? Strong, but sensitive. These are direct opposites which create dramatic tension.
Except they don't, really. Maybe they once did, and given enough spin on them they still can to some extent, but these days the whole idea is so horrifically cliche it makes my spin curl. Say what you want about Thunderbird III, but at least he spent the time he wasn't worrying about killing people with his awesome power trying to fuck everything that moved, rather than moping like a bitch. When Piotr isn't mooching about over not being any use to the team because he can't actually do anything without squishing people like ants, he's fucking up women left, right and centre because he can't distinguish between being naive and inexperienced, and just being a total fucking tool. Every time Colossus appears in the same panel as Shadowcat, or any woman he has any interest in, the subtext starts screaming "LOOK HOW HARD IT IS FOR THE METAL MAN TO BE FLESH AND BLOOD EVERYONE", and I lose interest. It doesn't help that most of it is his own fault. The man is incapable of seeing anything from anyone's perspective than his own, a fault that over the years has led to him joining Magneto's Acolytes and punching Pete Wisdom hard enough to almost kill him from cranial bleeding (the reason for this: Wisdom dared to be dating Colossus' ex, whom he split with after cheating on her).
I guess it's no surprise Colossus is an artist. An artist gets to imbue a piece with the exact emotions he wants, in the exact amounts. He gets, in effect, total control. The scales are his to do with as he pleases. I said you could draw a straight line between the two halves..
The one time that it all came together was on the day Colossus died. Years earlier, he lost his younger sister Illyana to the Legacy Virus. There was a lot of moping, of course, and the subtext started yelling again ("HIS AWESOME STRENGTH WAS USELESS AGAINST DISEASE HOW CLEVER IS THAT!?!"), but the horror of his loss led ultimately to greater understanding. He began to realise that there were other ways to fight for what he believed in than punching a hole in it. When an opportunity presented itself to cure the Legacy Virus at the cost of his own life, he didn't think twice. All he had to do was inject himself with the cure and activate his mutant power. Jump all those gaps, from flesh to steel and plague to cure and life to death. If you can't make your jumps smaller, you have to make your goals further away . That storyline was probably the first time Colossus got anywhere beyond pointless-to-irritating on my patented tolerance scale of X-Men. As this series of posts have ably demonstrated, that isn't an easy thing to do.
Of course, all of this was undone by Whedon of course, the great big bastard. Seriously, in general I'm all in favour of the Big W (as he likes to be called), but bringing back Colossus pissed me off in the extreme. His sacrifice was so perfect, the ultimate fuck you to both his image as being nothing by a whiny powerhouse (which, admittedly, I have embraced throughout this post) and to the quiet little virus that killed his sister, and which had been ravaging the Marvel Mutant Universe for around a decade of real time. It's not everyday you get to combine a satisfying ending with poetic justice and tidy bookkeeping. 
As far as I can tell, Whedon only brought him back to make playing with Kitty more fun, before apparently killing her off. For a lot of comics fans, that's like bringing back Jason Todd and involving him in a chain of events that ends up killing Batman (what's that? Well, fuck you, DC). Since I don't really like Kitty either, though, for me it's more like resurrecting Jason Todd and involving him in a chain of events that ends up killing, well, whoever all these fucking people are. It still bugs me, though, because curing the Legacy Virus took Colossus' years of service to the X-Men and both justified it totally and ended it perfectly, and I hate having a perfect ending ruined by a sequel. Even if Whedon had written it, I don't want to see American Beauty 2: The Return Of Lester.
As long as he's back, though, there's hope that Colossus will find a way to line up his gaps and his jumps once again. Once he's finished sulking over Kitty, obviously.
Next time round: we finish our consideration of the Sixties (update: by which I mean Seventies, obviously, what with understanding the passage of time and all) X-Men by asking ourselves whether or not the wrong strongman got blown up in mid-air like an itty bitty bitch?
 Off topic, but this is why anyone who tells you that belief in God is necessary to see a person as more than a collection of molecules is full of shit.
 Scott Lobdell: misunderstood genius. Discuss. Not too hard, obviously; the answer being clearly "no", though he is somewhat under-appreciated.
 Also: having his body stolen from the morgue, replaced with someone else's, and then brought back to life by an alien in an attempt to pre-emptively fuck with the X-Men. Lame.