I'm excited about ActiveJob because, finally, people will be able to build background jobs on things that aren't Redis. No hate on Redis - I'm a huge fan of it - but sometimes you have stronger consistency reqs that Redis does not have, and stronger availability reqs that SQS or PG don't have (aka multi-region jobs).
If the concurrency in ActiveJob become as good or better than Sidekiq, we can really get a lot of interesting stuff out there.