[ghc]

I've only been through a few cycles, but there's three flavors I've seen:

1. Better query language: QUEL, LINQ, etc.

2. Better data model or performance: CouchDB, Mongo, Redis, etc.

3. Better abstraction: Zope, Active Record, et. al.

SQL vendors keep absorbing the differentiating features from other approaches with enough success that they capture most business use cases. That's not to say there aren't successes outside of SQL! But I've seen it claimed SQL will be dead several times over thanks to some new tech and most of the time SQL instead kills that tech or makes it niche.