I agree. Russian is the most conservative Slavic language in most departments, including vocabulary, tenses, cases, pronunciation, etc. The only important way in which it's not as conservative as some other languages is that it has a lot of loanwords, but there are many synonyms for almost every loan word
1. Russian is comparatively conservative regarding the consonant system, but is probably most innovative regarding the vowel system, at least if we exclude some peripheral northern dialects. The vowel system is the main factor which makes the spoken Russian language particularly poorly intelligible to most other Slavic speakers which weren't subjected to it beforehand (that and swallowing whole syllables in particularly frequent words when the context gets only slightly less formal
2. Loanwords are a tricky matter. The modern Slavic languages with smaller shares of loanwords simply have those replaced with puristic neologisms, but purism isn't the same thing as conservativeness.
3. If anything, the most archaic verbal system belongs to Bulgarian and Macedonian. Russian is pretty mainstream in that regard - which means highly innovative. How can one seriously call Russian "most conservative regarding tenses" if it has lost the aorist, the imperfect, the past perfect, and has turned the present perfect into the past tense, much like all other West and East Slavic languages have done?..
4. Regarding nouns, Russian has preserved the core of the case system, which is, however, also a pretty mainstream thing. It has also lost the dual (unlike Slovene) and the old vocative case (unlike many other languages, East Slavic included). Finally, it has developed some new (mostly marginal) cases, which is an obvious innovation.
5. The cardinal numeral system contains many archaic elements, but in general that complex and irregular monstrosity comes from Early Modern Russian, with considerable changes in syntax and morphology compared to the earlier state. Hardly can really count as a conservative feature.
6. Russian preserves the old opposition between nominal and pronominal forms of adjectives, but their meaning has changed completely; the old pronominal forms are now the default ones, while the nominal forms have become predicative (with numerous limitations). If I remember correctly, Bulgarian and Macedonian are the only living languages with similar traces of the former opposition; they have basically preserved the original usage, but morphologically the system is completely reworked. It may count as a tiny bit of conservativity, I suppose.
All in all, I'd still say that Russian isn't particularly conservative by Slavic standards, and that the very task of finding the most conservative Slavic language has little sense.
Experiment. The speaker reads the text. Each person of the public writes down the translation into his/her native language.
I must note that it wouldn't directly indicate conservativeness of the language; it would only indicate how well its speakers understand Old Slavonic, which isn't the same thing at all. For example, Russian would have some advantage here because of its comparatively archaic orthography and pervasive loanwords from Church Slavonic; however, neither can really count as a sign of conservativeness.
Ah, I see. Thanks for the info. I don't think it would be realistic to expect Russians to know what this word means
They simply would have a very hard time recognizing the Russian /nasuɕnyi/ in the Old Bulgarian /nasǫštĭnyi/. I doubt it would be easier for any other Slavic speakers anyway.