The solution's grammar and syntax should be easier.

It does take a little bit of effort to use and understand the solution. It would help us a lot if the solution offered up more documentation or tutorials to help with training or troubleshooting.

MongoDB can realize more complex operations than Solr can. Solr should add some more complex operations to the database to at least bring it up to MondoDB's level of functionality. It would make it more competitive. 

There might be some compatibility issues between the data types within Solr. This needs to be improved.

Solr has a schema that we have to load the schema as an HTML, or SML file. This usually needs to be done by our engineers. It should be easier to do without needing too much technical background.

The performance for this solution, in terms of queries, could be improved.

Improvements with the backend and capability could be made so that it is easier for the engineers to maintain it. 

Memory utilization could be better but it is an industrial strength tool so some overhead is to be expected.

SolrCloud stability, indexing and commit speed, and real-time indexing need improvement.