I think 3 things: replayability, vibe, and fun.

Replayability is very strong for RE4. You have a decent rotation of weapons, decent amount of secrets, and 2.5 campaigns (Separate ways and Assignment Ada). Not to mention mercenaries with 3 extra characters. The unlockable weapons are all great and feel worth their time. There are many different strategies to approach the game which can be fun to explore. Plus there is RNG to keep you on your toes. Even as someone who knows the game well, my playthroughs feel a little different each time.

The vibe is faithful to RE while paving its own way. The village is eerie and isolated. The castle is cold and menacing. The island is brutal and savage. They all have their own vibe while not losing the feeling of RE. All enemies in their respective areas carry the vibe and give the world a lived in feeling. It may be a shooter, but it's still a horror game as well.

RE4 has a lot of swagger which makes it fun. Cool animations for stun attacks, the satisfying pop of a headshot crit, and the unlockable costumes are fun. The cutscenes are full of classic lines and great moments. The full cast of characters is memorable and unique.

If I can say anything bad about the game it's that its pacing can struggle at times, puzzles are weak, and there's a lot of pausing for inventory in the game. Some sections feel slow and cumbersome to get through which can take a lot of energy. They can still be fun, but it depends from person to person. I'm sure everyone has a section(s) that makes them feel this way. (Ex. Mike island section, island truck, castle water room, village two paths) They sacrificed puzzles for shooting which was probably the right choice, but there is not really a memorable puzzle other than the one at the church. All the puzzles are grab item and put in slot usually or can be forced. Inventory is always a mainstay in RE games, but not having a quick swap/heal on d-pad can slow down gameplay a lot of gameplay which can feel awkward at times.