Incremental and partial Java compilation. Partial compilation is the notion that partially compiled software is still usable and something you can debug or introspect on. The IDE compiles while you type, it will detect syntax problems and as soon as you fix them they go away. Error markers in intellij are nowhere near as fast and you can't trust their presence or absence to be correct either.

Intellij does that a little bit of incremental compilation. They half integrated the eclipse Java compiler at some point but they never really made using it a thing. Most projects just hand off compilatio to Gradle or maven, which means you lose a lot of time waiting for that and dealing with various caches in intellij and gradle/maven getting out of sync with each other. The process for fixing that generally involves boiling the oceans and running the CPU at 400% for a few minutes.

Eclipse compiles two orders of magnitudes faster when you have it properly setup. I'm not exaggerating. It's really good at this. Intellij users have never seen this level of performance. I've tried to explain it a few times to users that insisted they had intellij configured just right. The minimum execution for a unit test after a 1 character code change is the key benchmark for this. Run a test, change something trivial, run it again. If that takes more than half a second and has a noticeable delay, it's not fast. Eclipse user to be closer to 50-100ms. Even on a (now) ten year old laptop. With intellij, it's closer to 3-5 seconds. They have improved things over the years but it never got even close to that level of performance.

Compilation of everything, how it shows compile errors and warnings. I see them in tree, in list, easily and quickly all of them. All of them for whole project at once and trustworthy. Less bogus warnings and contraproductive suggestions.

I never needed to fiddle with "command line shortener" ridiculousness in Eclipse.

Better code navigation and discovery. Eclipse always finds everything, every called and callee to depth. Same with inheritance. Even where Idea has same views, for some reason they hide it and only former Eclipse users stumble on it.

Actually safe refactorings. Idea occasionally "refactors" by creating bugs.

I had some cases where Idea simply formats code wrong. And it was impossible to configure to formát it right.

Also, Eclipse compile by default. It is ridiculous to have to remember to press shortcut to compile in modern IDE. I know it is configurable, but still ridiculous default.

Square selection.

Elipse keeps a complete Java representation of the source code in memory, (not the classes) that's why it can find compile errors before saving. This comes at the cost of lost of RAM, some projects are just too big for eclipse so IntelliJ is your only option. But that eclipse model can be used for lots of cool tricks, you have access to it in the plugin API, so for example you can enforce coding standards before even saving code.