The 3 concepts or habits I find my students struggle with the most:
1. Reading a word problem carefully and pulling out all of the necessary information and what they are being asked to do with that information.
We spend a lot of time discussing how to translate a problem from words into math, and to make a plan for solving it. I think the process of breaking down the problem into pieces and translating each one is really useful, but no matter what I do, it's hard to get kids to actually apply this.
2.. Working in groups.
We have been using Complex Instruction (CI) at my school over the past year. CI helps train students to work together in groups by fulfilling specific roles, working on complex problems that are "group-worthy" and trying to improve the status of students who aren't the typical, "good students" by finding what they do well and pointing it out to the group. It is hard to really follow the rules for complex instruction all the time, but if you do it really makes a difference in how kids work together and how they approach complicated problems.
3.. Showing their work or explaining their thinking
The Common Core math standards depend on being able to explain your thinking, and this is the hardest thing for all students to do. In fact, very strong math students often have a harder time with this than students who need to think things through more explicitly. Complex instruction helps, making students answer all word problems in complete sentences helps, scaffolding helps, and I also find that praising kids not for getting the right answer, but for explaining their thinking also helps.