what exactly is present in intellij that isn't available in vscode / neovim with lsp / newer emacs? I ask because, I use(d) intellij for java and maybe i'm not using most of its capabilities or they're hidden away in plain sight, but i haven't noticed anything special aside from the slow ui.

For me there are two axes that are lacking (based on Scala and Elixir usage):

1. The feature set is just far weaker. If an LSP implementation and the client both hit 100% coverage, then you might end up having the same refactoring, code generation, follow definition/load docs/infer typing support. But of course the Elixir LSP matrix shows that basically none of the LSPs offer a complete feature set and you basically get to pick which features are missing. What about debugger support?

2. This could be viewed in some ways as a recapitulation of point one, but I actually appreciate the Integrated in Integrated Development Environment. I very much dislike when adopting a new technology having to cobble together a disparate number of tools to create a good editor experience, especially in a time where as a novice I'm actually not at all qualified to understand what a good editor experience would be! I actually call this the Clojure Problem, which is that many Clojure devs decide they have to learn emacs or a complicated fireplace.vim setup in addition to Clojure and wind up learning neither.

Regarding speed, don't get me wrong, I empathize with complaints about it and whenever I'm doing C# work for one of our services I'm reminded of just how slow developing Scala can be in particular. But I don't think of LSP approaches as fast, either. Perhaps they are async and don't block the UI, but now I'm just sitting here typing and getting constantly out of date feedback from the editor as it and the LSP catch up to whatever text has been entered.