One thing I've found tricky to learn in Chinese is the numbering system. Every language has their own way to name the numbers and numeric concepts, but Chinese is the strangest to me.
First of all, there are no distinct words for numbers bigger than 10 (十), until you get up to 100(百), 1000 (千), etc. This makes it easy to remember but conversely more confusing to hear for me. For example, the word for "twenty" is actually pronounced as if you were saying "two ten" (二十) in English. But this means that a word like 74 is prounced "seven ten four" (七十四). This is fairly logical and easy to understand, but because I'm still getting used to this I have to first hear the individual numbers and then recombine them in my head to form their actual values.
It gets harder as you get to larger numbers. First, there are words for 100 and 1000, but there is also a word for 10,000 (万). 100,000 is ten-tenthousands (十万). This means that you have to group numbers by 4 digits instead of 3: The number 300,223 isn't "three hundred thousand two hundred twenty three", it's 30'0223 - "thirty ten-thousands two hundreds two tens three" (三十万二百二十三). This makes it hard to convert the numbers unless you've totally internalized the different representations.
Oh, and you usually don't use er2 (二) to say "two something", you have to use liang (两). So if you want to say "Two dogs", you don't say er zhi gou (二只狗), you say liang zhi gou (两只狗).
Then there's the convention used for talking about discounts. In English we often talk about how much on sale something is: 25% off, for exmaple. In Chinese you say how much is left to pay: da qi zhe (打七折) would be "on sale for 7 tenths." That means it's 30% off. If you want to say 25% off, you'd say 7 tenths 5, or da qi zhe wu (打七折). As far as I know, you can't sell something for more than 90% off: 90% off would be da yi zhe (打一折), or 1 tenth, but you can't say da ling zhe jiu (0 tenths 9)(打零折九).