Focusing on the main problem/issue
What is the main problem?
What is the real issue (here)?
(I think) the major problem is . . .
Our primary concern is . . .
The crux of the matter is . . .
(As I see it), the most important thing is . . .
The main problem we need to solve is . . .
We really need to take care of . . .
It all comes down to this: