I don't think it affects the meaning that much unless the word you're not using the kanji for has two meanings but I am pretty sure it all depends on the context. It may only make the sentence harder to read but that's all about it. Personally, I've never had much trouble reading sentences in which kanji were not used though.

かんじ

ひらがな　だけで　ぶんしょう　を　かくと　とても　ながい　ぶんしょう　に　なって　しまいます。

また、とても　よみずらい　ぶんしょう　です。

いちばん＝一番

ひらがな≦かんじ

ひらがな　は　おと　だけですが　かんじ　には　いみ　が　あります。