Surprise is a really good point and something that we can generally apply.
But I am not a fan of the theme/brand point. Lots of people say it, and it seems easy to say once a company is successful - but hard to figure it out when you are in the early stages.
You as the product creator have to make decisions about what your product means, what it stands for and how it will come across. Imagine your product was a real person - how would they behave? What would they say? Those are the kinds of things you'll then need to build into the product.
No one said it'd be easy, but I think it's worth it.
I did not mean to say that I don't like doing it - just that I have heard it so many times and that making it happen is hard. I would love to hear more people breaking this branding thing down.
I see that you're behind architexa and I can see you're not sure what kind of personality you want for your product. Why did you decide to make this? Is this a personal problem you have? Who is your target customer? Why do they want your product? Do they want to work faster? Or prevent mistakes? Or use something with very little friction? And don't say all of the above, because no product can be all things to all people, just like no person can be likeable to everyone (well, except Ryan Hupfer).
Answering those questions starts you down the road of developing your product's personality. Also check out 37 Signals's article on finding an enemy - http://gettingreal.37signals.com/ch02_Have_an_Enemy.php
If by brand you meant your logo, consider that your theme and your logo will be for sure noticed by your users/visitors and what they communicate will be taken as part as your brand and personality.