By far, it is managing cash flow. You need to be VERY good at it. There is an extremely fine balance between the speed you can invest and the speed at which you get new customers (i.e. revenue). And it is very easy to tip it of one way or another.
If you are too risk averse, you will not invest enough and grow too slowly and be overtaken by market, if you take too many risks, you can be broken and have to shut down ... as said, very fine balance!.
But mastering this balance will equip you with extremely useful skills that will prove invaluable for later stages of your startup and that, from my point of view, people that try to get funding as soon as possible don't fully develop.
So face the challenge with a brave face! :)