If MySQL is giving you more troubles than it solves, I'd take a look at MongoDB, CouchDB or ElasticSearch (depending on your use case). Maybe even Neo4j. Your choice should come down to several points such as replication, scaling capacity, consistency... I advise you to read carefully some official documentations before you decide. Here's a starting point for comparison.
Going NoSQL will get rid of all the joins and improve your performance but you'll still need to implement a proper hierarchy using adjacency list, nested sets, materialized paths and such...
Keep in mind NoSQL technologies above pretty much all use eventual consistency, which essentially means that your data might not be consistent at a given time among some nodes. If this is a problem you should stick to RDBMS.
