How well multi-player works.

If it is coop, like in Bubble Bobble(1986), that needs occaisional partner assists or straight-up player saves.

Or competitive, like Wipeout 2097(1996), which has excellent catchup mechanics.

Or larger scale competitive, like Quake1 shareware(Qtest) at 16-player.

Each of these showed examples of quality play-testing and balancing, making for enjoyable player interaction.