Pro Enterprise strength security and transactions
The two fatal flaws of many open source solutions is lack of integrated security at the element level and full ACID transaction support. MarkLogic has both and is trusted with some of the worlds most sensitive data. It is the engine that powers healthcare.gov, which despite some early problems (not caused by MarkLogic) is an amazing technological achievement.
Pro Multi-model database
MarkLogic supports text, documents, key-value/tuple, graphs, tables and object models that don't require extensive data-modeling and normalization that is part of the lifecycle process of relational database management systems. For sites similar to LinkedIn, Facebook, IMDB and even search engines, MarkLogic provides a unique set of features that are all in one box.
Pro No ETL Required
MarkLogic can store all your enterprise data in it's original format without needing to know a schema in advanced. You can shove pretty much any structured or unstructured data directly into MarkLogic, and it will automatically index everything and make it available for future processing. Of course it is fully schema aware and will apply and enforce schema constraints when available, but the tedious normalization that is required for relational databases is not necessary.
Con Enterprise software is not open source and can get expensive. Not for casual projects.
MarkLogic can be downloaded and implemented in development environments for free. However, for production use, it is priced for enterprises, not startups with tight budgets. Open source requires a lot more elbow grease to do the same thing.