"Warmer and more conversational" - they're basically admitting GPT-5 was too robotic. The real tell here is splitting into Instant vs Thinking models explicitly. They've given up on the unified model dream and are now routing queries like everyone else (Anthropic's been doing this, Google's Gemini too).
Calling it "GPT-5.1 Thinking" instead of o3-mini or whatever is interesting branding. They're trying to make reasoning models feel less like a separate product line and more like a mode. Smart move if they can actually make the router intelligent enough to know when to use it without explicit prompting.
Still waiting for them to fix the real issue: the model's pathological need to apologize for everything and hedge every statement lol.