For example, the distinction between миръ, peace, and міръ, world, was lost. Also lost: the distinction between ѣсть, to eat, and есть, to be. There were plenty more distinctions lost in written Russian as a result of the Bolsheviks' ill-advised spelling reforms. I grant that ѵ was only used in a few words of religious import (сѵнодъ, мѵро), so perhaps replacing it with и made sense, but the loss of the other letters destroyed meaning. Анаѳема became анафема and names like Ѳедор and Ѳома became Федор and Фома. I could go on and on.

Even something as "simple", so to speak, as the loss of word-final ъ removed an entire understanding of the "бѣглый ерь". Counting from the end of the word, you drop all the odd-numbered "yers" (ъ or ь) except the very first one (i.e., the word-final "yer" ) and lengthen the even-numbered ones to e (in the case of ь) or o (in the case of ъ). Thus, the Slavonic form дьнь becomes день, but the Slavonic дьнь сь ( "this day" ) becomes днесь (as in the Lord's Prayer: хлѣбъ нашъ насущный даждь намъ днесь) because the postclitic demonstrative adjective is counted with what it modifies.