All the extra stuff that makes vim more than just a vi clone. Everything from built in stuff like the auto-completion to plugins like syntastic. Everything technically works, but never nicely and I always end up frustrated with the warts and just go back to plain vi. I should give emacs a shot, I've heard evil is supposed to be a better vi than vim.
Other editors/IDEs you are using with vi-mode plugins also have extra stuff that make them more than just a vi clone, from built in stuff like auto-completion to syntax checking plugins. etc.
Of course, I want those things, that's the whole reason for trying to switch from vi. I just want those things to actually work smoothly and together, rather than being a mess of plugins that sorta work sometimes but have lots of ugly warts and don't play nicely together,