What are the most valuable features in your trading software?

I work as a coder, and i’m experimenting trading since 2 years.
I’ve always sticked with MT4 for my trading, until i felt the need to have more customization, and since the programming language i know best is Javascript, and i didn’t want to learn MQL for scratch, just to use it on MT4, i decided to write my own trading software, tailored on my needs.
The more i go deep in the process, the more i ask myself if i’m missing something looking at things only from my point of view.
[B]So i would like to hear from you guys, as per title, what are to you the most valuable features in a trading software, and if you like, why.[/B]
I’ll start myself, for me it is all about:
Great flexibility that allows you to implement even the most complex strategies, and to trade out of the ‘standard’ schemes.
And of course, the software must be robust and reliable.