Vem differs from Vim in three main aspects:
Key commands: Vem key command set is smaller, simpler to use and distributed spatially across the keyboard. Vim command set is much bigger and many commands have a syntax that allows you to compose more complex ones by adding counts, movements or text objects. Vem commands are all about simplicity while Vim ones are about expressiveness.
Default configuration: Vem default configuration tries to make the editor more intuitive without being too opinionated. Things like the clipboard, cursor movements and the user interface work much more closely to what you would expect in modern editors than in Vim (which mostly values backward compatibility).
Additional plugins: to support the previous two points Vem includes extra plugins. Vem doesn't try to add popular plugins to make the editor more complete but only to add those that are considered essential for Vem's own functionality. In Vem, like in Vim, you have the freedom to shape the editor to your liking by adding the plugins that fit you. Check the Plugins section to know more.
Square bracket, z and g commands
Square brackets ([ ]), g and z are keys that serve as prefix to many commands in Vim. While these commands are not part of Vem's command set (except a small subset of z ones), Vem doesn't prevent you from accessing them.
The square bracket commands are associated with more complex movements and jumps. Some of them already provided in Vem. You can use them as you would do in Vim. To know more about these commands, check:
The g and z commands are pretty varied and can't be really be put in a single category, although most z commands are associated to operations with folds. You can access these commands, but they are remapped in Vem:
same as Vim's z
same as Vim's g
To know more about them, check:
:help g :help z
As you can see, the commands listed in the folds section are just a subset of the z ones using its b mapping. If you want to use the full range of actions for folds, they are available to you.
Note that, while you can access all the commands associated with [, ], g and z, you can't provide counts to them as you would do in Vim.