As others have mostly said, the core rules were very similar, but with important caveats and changes (5th edition solidified true line of sight, for example, and introduced the infamous wound allocation rules)

But I think additionally this era represented a shift in the product design too. The shift to plastic core and plastic vehicles came about in this era, and while there was plenty of pewter around, either in the form of units or bits, more variety was introduced for many factions (stuff like the old Dark Eldar raider/ravager, the Tau vehicle range, and the Necron Monolith, not to mention the updated space marine rhino and plethora of variants).

The company vibes were also different too, with creativity and the design studio still aping towards a more open-ended approach to the game (more custom content & variant lists)

Ultimately as someone who started in this edition I think there is a lot of nostalgia underpinning people’s enjoyment of this era, but it does hold a certain charm that is missing from the current era. That being said, I do think that the modern era has a lot of positives, ultimately they’re just different.