In the long-term, how different do you want to be from Apache Pulsar? At the moment, many differences are obvious, e.g., Pulsar offers transactions, queues and durable timers.
(Founder) We want S2 to be focussed on the stream primitive (log if you prefer). There is a lot that can be built on top, which we mostly want to do as open source layers. For example, Kafka compatibility, or queue semantics.