What are some similarities between Indo-European languages?

When some people say Spanish and English are more similar than Spanish/English and Japanese they often argue it is because the first two are Indo-European. But what similarities do these two languages have in common? I mean if their syntax/vocabulary/grammar/phonetics are similar in some ways. I know that English and Spanish share lots of words derived from Latin, but Latin is not their common ancestor, right? So, what these two languages have in common, and are these things useful when a Spanish-speaker is trying to learn English or vice-versa?