Mystique can only take on the forms of others, not their powers. From Wikipedia:
Mystique is a mutant shapeshifter with the ability to psionically shift the formation of her biological cells at will to change her appearance and thereby assume the form of other humans and animals. She can also alter her voice to duplicate exactly that of another person. Originally, it was clearly stated that Mystique's powers were limited to appearances only; she could not assume the powers of the people she morphed into or alter her body to adapt to different situations. Additionally she could not change her overall body mass when taking on the appearance of a person larger or smaller, but thanks to subsequent enhancements she has stated that her body mass is not fixed and can change when she does.
As such, she clearly can't completely mimic Colossus' metal skin, or, as phantom42 mentioned, Wolverine's adamantium claws. She could easily mimic the appearance of both, and has mimicked Wolverine on occasion. With her powers increased after exposure to radiation, she is not now able to mimic the appearance of certain animals, like dogs, and can flatten herself paper-thin (see previous link for details). It seems like they have really gone out of their way in the last ten years to make her more like the T-1000 in Terminator 2: Judgement Day (she can turn her limbs into bladed weapons), which is a change I personally dislike.