- Ease of setup and install. Turnkey. Good defaults.
- Non-buffer based workflows.
- Easy access to settings.
- Easy ways to change or switch my compiler.
- Integrated with typical lisp tooling for library, system, and package management. (For example, what Emacs button do I press to set or clear my ASDF compile cache?)
- Better integration of the profiler and debugger. When a Lisp error happens, yet another buffer pops up (breaking the arrangement of all your code windows you set up), this buffer may not even be the only one (but the others are hidden somewhere), and it's not clear what you can even click or expand to see more information (there's a tremendous amount, extremely non-discoverable).
- Good getting started: built in guide for structural editing, REPL workflow, etc.
...and much much more. I say all of this as someone who basically has only used and invested in Emacs for 20 years. I love sharing Emacs with people who like weird technologies and rabbit holes, the real "hacker"-type people. I hate sharing Emacs with people who want to be productive in an hour or so with a Lisp project, because I know within 5 minutes they'll be disappointed, and never get the best of the experience because it's too much uninteresting investment.
I prefer writing Lisp with Emacs+SLIME over anything else. It's extraordinarily powerful, and with enough grit, you can get it to do almost anything you want. But I'm also jealous of people who get to use, say, polished JetBrains products whose goal is to try to give you the best experience possible for your specific programming language.
JetBrains IDE plugin for Common Lisp: https://github.com/Enerccio/SLT (I'm sure you saw it before and I don't know how polish it is, and I'm pretty sure it has less features than Emacs&SLIME, yet, but I must link it for reference. Because yes, before 2023 we could complain there were no JetBrains IDE plugin for Common Lisp, since 2023, we have one.)