Your question is very interesting for me! I'm Japanese and study Spanish helping "amigos" who are learning Japanese. I'll write what I learned until now by communicating with my "amigos". I think the pronunciation between these two languages is very similar although they aren't totally the same, and except some. It's hard for us R de "respuesta / desarrollar / rodilla" y la diferencias entre NI and ÑI, sometimes J, too, like "reloj". My "amigos" say that it hard to distinguish the Japanese pronunciation of Z and S (ざじずぜぞ and さしすせそ), Y and J (やゆよ) and (じゃじゅじょ), T and S (ち and し / つ and ちゅ), and the long vowels and small TSU (っ), too.
About grammar, it's easy to understand the difference between TÚ and USTED for having different levels of politeness but we have more.
Until now, when I explained about Japanese, it's easy for "mis amigos" to understand some because:
You can omit the subject and we can do the same depending on the context.
Japanese is flexible like Spanish is, and we can change the word order like you can do.
Sometimes there are cases which the sentence sounds unnatural though it's correct, and sometimes it depends on the person/speaker and it often happens in Spanish.
What I always say to Spanish speaking learners is "there are many differences between our languages, not only for us, either not only for you". Because:
You have a lot of conjugations, subjunctive and several ways for "past", you have very complicated pronouns.
We have kanji, keigo, and Japanese counter.
*Todavía mi inglés y español son muy lejos de con fluidez. Espero que haya sido entendible...
Si hay algo que te ayude o si tienes duda en mi respuesta, me dices. Espero que haya sido útil mi respuesta y opinión. ¡Mucha suerte!