Main Advantages of rsyslog v7 vs. v5

greatly improved configuration language – the new language is much more intuitive than the legacy format. It will also prevent some typical mistakes simply by not permitting these invalid constructs. Note that legacy format is still fully supported (and you can of course do the same mistakes if you use legacy format).
greatly improved execution engine – with nested if/then/else constructs as well as the capability to modify variables during processing.
full support for structured logging and project lumberjack / CEE – this includes everything from being able to create, interpret and handle JSON-based structured log messages, including the ability to normalize legacy text log messages.
more plugins – like support for MongoDB, HDFS, and ElasticSearch as well as for the kernel’s new structured logging system.
higher performance – many optimizations all over the code, like 5 to 10 times faster execution time for script-based filters, enhanced multithreaded TCP input plugin, DNS cache and many more.

Of course, there are many more improvements. This list contains just the most important ones. For full details, check the file ChangeLog.