There are many things I like about Grunt, and many of them are still true.
I like that Grunt gives you the choice of being a Config file or just straight up JS (write your own functions using Node Modules).
I like that it had it's own API to do things and forced plugins to be written in a common way (much like jQuery) - this made it easy to A : Use plugins quickly; and B : Write plugins.
I like that it garnered a quick following and is now used by a lot of Open Source projects. This in no doubt helped spawn a metric-shitton of plugins.
But there was room for more. There's always room for more.
gulp.src takes a glob (a list of files) and then you just pipe them through plugins. This is the thing with Gulp, it's all about Streams. Just think of it like the chaining you do with jQuery.
I like Gulp a lot, it feels like discovering Grunt all over again. I like the way its code over configuration; don't get me wrong, I love the simplicity of Gruntfiles and I think it gives opportunities to those who wouldn't normally write their own scripts to assist with automation to dive in. I am also aware that with Grunt you can choose to have code over configuration.
Grunt and Gulp can co-exist to serve different groups of people, for now though, im sitting with Gulp. I can't wait to start doing more with it.