"Some" differences. OGL ES 2 is more like a mix between ogl 2 and 3 (still not supporting all features of version 2) and imho way more different then "some" differences.
It uses its own shader language for some totally crazy and stupid reason.

I've only learnt 2.0 (with a focus on ES and how to use the desktop version in a compatible way). The only differences I'm aware of are the #version number and the precision specifiers. These can easily be taken care of by adding the appropriate #version and #defines for the precision when compiling the shaders. But I've also glanced at some examples for 3.x and this seems to have quite a different dialect of GLSL from 2.0, e.g. the 'layout' keyword.

The Api names are similar but the architecture how it works is different and at least in my case it forced me to rethink the whole graphics architecture of my program when porting it.
Of course this was my case, maybe your software base was coincidentially suited for OGLES but in my case it was not :/

I should think it's harder to start with non-ES and convert it to ES later than if you keep ES in mind from day one. And it sounds like you were pushing the limits more than I if you had to optimise your code for different underlying architectures.