Articles. Not only there are none in Russian, but there is nothing that serves their function.

Happens all the time:

-- I just put my towel to laundry.

-- Okay.

-- But I just realised that I need towel again. Could you go fetch towel for me?

-- Here, I brought you towel.

-- This is another towel.

-- Oh, so you needed that very towel that you put into laundry?

-- Oh. (switching to English) I wanted to say "I need the towel", not "I need a towel"!

Next, Russian often requires you to specify a lot of extra info, compared to English. Example:

-- Why is that thing a fish?

-- It isn't. (because it's a dolphin)

"It is a fish" = "Это рыба" (it fish). No 'is' in this sentence in Russian. So, instead of "it isn't" you must say "it isn't a fish". There is no easy way to say this sentence without using the word "fish" or some extra clumsy wording like "not the thing you are asking about". That makes it very hard to make stuff like chatbots in Russian, or write generic lines for RPG games where the same line can be used in different circumstances.

Same thing with grammatical genders. When you say "X does Y", you must specify gender of X in Y's form. A lot of media was botched in translation, when one character thinks that another character is a girl when he's actually a guy (and is not trying to deliberately deceive). In Russian, it is hard to say more than a couple of sentences without revealing your gender in the process.

Is that enough? There is more where that came from.