Forgetting the color and patterns because both cats can have a wide range of colors and patterns, what strikes you as the obvious difference in appearance?
It has to be the ears. The Maine Coon ears are larger and more pointed which with the ear tufts creates a completely different appearance to the smaller more rounded Siberian cat ears. The breed standard for ears for these breeds is (verbatim for accuracy):
- CFA: SIBERIAN EARS: medium-large, rounded, wide at the base and tilt slightly forward. The ears should be set as much on the sides of the head as on top. The hair over the back of the ear is short and thin. From the middle of the ear, the furnishings become longer and cover the base of the ear. Ear tipping is allowed.
- CFA: MAINE COON EARS: Shape: large, well-tufted, wide at base, tapering to appear pointed. Set: approximately one ear’s width apart at the base; not flared.
Another difference that you can't see in the pictures by Helmi Flick (images are copyright protected, please note), is that the Maine Coon is larger than the Siberian. On my assessment the Maine Coon is the largest purely domestic purebred cat.
I won't quote the breed standards anymore but highlight some general differences in appearance, character and history.
Coat: Siberian: a long thick coat with a tight undercoat. The coat is referred to as triple coated. The hairs are of similar length. Maine Coon: uneven shaggy coat is not dense or full.
Muzzle: Siberian: shorter than that of the Maine Coon. It is full and rounded. Maine Coon: broad and square muzzle.
Head: Siberian: large and impressive modified wedge. Medium in size. Maine Coon: broad modified wedge. The head shape is similar for these cat breeds as confirmed by the photographs.
Conclusion: the Maine Coon has larger and more pointed ears with a slightly more slender face and a slightly less neat coat. It is also a larger cat. The tail in the picture is more plumed for the Maine Coon than the Siberian but this may be just an individual cat difference.
See: Siberian breed standard and Maine Coon breed standard (links to cat association standards).
Siberian: strong minded and independent and therefore likes plenty of living space. Dog-like devotion to human companion. Maine Coon: a relaxed cat and relatively easy going. The Maine Coon is not overdependent on people and good with children and other companion animals.
Siberian: native Russian cat. Thought of as an ancient breed. Let's say that this cat was around as a cat companion about 1000 years ago in Russia. Obviously at that time it was not thought of a breed of cat. It was a random bred cat. Maine Coon: I think that the history of the Maine Coon is more interesting but that may be because we know more about it. This is a semi-longhaired cat that originates in the state of Maine, USA. They were farm cats before becoming show cats in the mid-1800s, when the first cat shows in the USA took place. The early origins are probably in the imported long haired ship's cats from Europe that accompanied the first immigrants to America. These long haired British cats where Angoras. Angoras were probably Turkish Angoras. Turkey is near the place where the first domestication of the wild cat took place (fertile crescent). The Maine Coon may have origins that are better connected to the absolute beginnings of the domestic cat.
The Maine Coon is ranked the most popular (1st) on my 4,500 vote poll out of over 60 breeds. The Siberian is ranked about 21st. This may partly be because the biggest cat fancy is in America (most visitors to PoC are American) and the Maine Coon is the American cat.