ZeroTier original (and still main) author here. Objectively speaking another difference is the ACL scheme which seems perhaps less powerful than our own rules engine but easier to use. The ACLs are closer to intents while ZT rules require some TCP/IP knowledge (like raw iptables or pf).
We have plans to address this ourselves more with a higher level UI to build rules from intents and common patterns. Also have more auth integration planned, but the list is long.
Significant performance improvements in the core, reworked CLI, crypto improvements, professional security audit of both design and code, easier root server federation, easier DIY controllers, better multicast, and of course bug fixes.
The firm doing our security audits is an extremely well known one. Don't want to reveal the name quite yet.
It's possible that some of that won't land fully formed right away in 2.0 but will follow shortly thereafter, but the performance, auditing, crypto, and multicast will be there.
With ZeroTier, the client connects to the network using a private shared key and then needs to be approved on the control plane independently.