Knowledge. Understanding what each step in the process does (or doesn't) do and how to troubleshoot. If a load isn't performing - what adjustments will make the difference? OAL, primer, powder, powder load, neck tension, bullet selection, crimp, sizing - knowing how each variable affects performance is important.
Time. Realize that all your work is being shot through "that" gun. Your perfect load will not be the same as mine, nor mine for you. Been reloading for 25 yrs or so but I've only spent 3 months in serious development. (was between jobs 20 yrs ago...) Tried a lot of other components since then when you could only get what was available, but never developed anything to match. Until I take retirement, I'll be shooting the same loads in the same guns and enjoying the same performance from them that I worked out long ago.
Tools. Meh, some help you go faster and some are more satisfying to use. But they are just the tools. Results are dependent on the operator.