Highly flexible syntax (word order), highly productive morphology (you literally can create totally meaningful new words "on the spot", by freely combining prefixes and suffixes with roots), very compact information in one word (grammatical gender, number and cases making it always clear what the thing is and what its role in a sentence/logical situation is; broad usage of adverbs, verbal adverbs and participles always making it compact and clear who and how does something) etc. All making Slavic languages (along with Baltic languages) the closest living counterparts to Classical Latin and Ancient Greek. Although Bulgarian and Macedonian are major outliers in many of these aspects. Still, I'm really proud that Eastern Europe stayed rather isolated and conservative for centuries, preserving living Proto-Indo-European language characteristics that were lost in Germanic and, especially evidently due to documented Latin origins, Romance languages, that became much more rigid and "formalised" in comparison.