In random order of importance:

• Splicing
• Better path-finding
• Huge improvement in payment success rates
• Realization that LN isn't perfect, yet is the best working thing we have
• Increasingly being used for large transfers between big players rather than the misguided early marketing focusing only on near-free small payments

I'd say the biggest development is that hype has been replaced by realism, which is usually a good sign as most maturing technologies go through such phases.