Off the bat, I am admittedly positively biased towards Chaosium thanks to shortlisting in their design contest. But in addition to that, the first game I ever bought as a young man was Stormbringer, and ever since I have voted with my wallet in Chaosium's favour.
So what do I like about their games? Four things:
1 Quality
Chaosium has always made good games. In terms of mechanics, tying everything to the D100 has been a major boon in terms of cohesion as well as simplicity. But even with things like art, Chaosium has consistently chosen evocative works. Even the early art, such as the handdrawn shieldmaiden of the original Runequest, was miles ahead of its competitiors, and has been creatively remade in the Mythus cover.
Even at their lowpoint in the late 90's to 00's, when Stormbringer was outsourced to Moongoose, there was a minimum of quality under which their games did not drop.
Speaking of Stormbringer...
2 Using IP
From the start, Chaosium made the smart choice of using an Intellectual Property (IP) as the game setting. Elfquest, Stormbringer, Ringworld, and Call of Cthulhu all had the groundwork already done by their creators. This allowed players to be immersed immediately in a fleshed out setting made by an established author instead of making (and forcing players to learn) their house setting.
In this way, Free League seems to be a spiritual successor to Chaosium, with their long list of evocative IPs (Alien and Bladerunner for movies, Things From the Flood based on art).
3 Personalities
Chaosium was run by passionate gamers like Greg Stafford, Sandy Peterson, and Lynn Willis. They were also very open - Lynn Willis sent me a kind rejection letter in the 90's, and Sandy let me interview him earlier this year. With gents like these at the wheel, make no wonder that Chaosium is known for epic adventures - Masks of Nyarlothep or Terror on the Orient Express for CoC, Rogue Mistress for Stormbringer. Once again, Free League seems to be following in their footsteps with some of the great material out there for Alien.
3 Letting others play in their sandbox
Whether it be subscribing to an open gaming license, or letting independent creators make manuscripts of award-winning scenarios for CoC, Chaosium has really invested in and benefited from community content. The BRP design contest is just another extension of this policy, which makes good business sense while avoiding the corporate misstep of shutting fans out for fear of losing revenue.
So I am glad to be a loyal Chaosium nerd. Now just to find playtesters for Nuna...