Quite large, actually. Church Slavonic lacks the nasal vowels and most of the reduced vowels of Old Church Slavonic, the tense and aspect system has been restructured, and some consonants have been changed to fit the norms of local dialects.
Just to clarify as someone who has studied Old Church Slavonic at University and used Lunt's text book:
The reason some may think Old Church Slavonic lacks vowels etc. is because originally the soft sign and hard sign were pronounced vowels.
Also for people who know Romance languages understanding Latin: modern Romance languages do not decline/ has cases for nouns as in Latin.
Romanian does. It's also the only Romance language that has a neuter gender
So vocabulary may sound familiar but the use of cases is confusing to those who do not know Latin grammar.
Totally off topic but being Canadian and learning required French from public school when I was in Romania I notice simularity in vocabulary but the articles are attached the end of the noun not set before the beginning of a noun as in French.
Yes, Romanian differs from the other Romance languages, but resembles the Slavic languages in the Balkans with a definite article, in this.