The differences do exist, and I'll try to explain them in further text.
First of all, one has to understand that Slavic speaking peoples are very heterogeneous in appearance, so we cannot speak about common Slavic appearance or race. They all look different to some extent, but there are also certain similarities. Russians are different than Croats, Croats are different than Macedonians etc. But the differences are in the "overall" appearance as a result of Slavic absorption(and assimilation) of different indigenous peoples, and there are certain similarities, resulting from common Slavic ancestors.
Look at this man, for example:
His name is Nikola Gruevski, and he's the prime minister of Republic of Macedonia. If you didn't know about him, and saw him on the street, would you ever say that he is a Slav?
I am not implying that every single Macedonian looks like him, but his appearance is common in the Republic of Macedonia.
And look at this man:
I'm sure you know who he is. And his look is common in his home country. Both of them belong to the Slavic speaking nations, but the differences are obvious.
Important note: pictures are used solely to illustrate the differences.
The languages are intelligible to some degree. I can understand most of their language if they speak slowly.
I wouldn't know about their way of life, but I know that every Slavic people has its own culture, and Macedonians are not an exception.
Simple.Because they are not Serbs nor the Bulgarians.