I agree - we (New Relic) have left the install process up to the plugin authors. Hence, plugin installation effort varies widely ; )
In the future you are likely to see some improved installation options, however because you can truly monitor _anything_ with New Relic plugins (at least, for now, anything that emits or can be made to emit time-series numerical data) it is unrealistic to expect that there will be only one, consistent install experience for everything.
In other words, installing the plugin agent to monitor your MongoDB will probably always be different from installing the plugin agent to monitor your John Deere tractor.
In the future you are likely to see some improved installation options, however because you can truly monitor _anything_ with New Relic plugins (at least, for now, anything that emits or can be made to emit time-series numerical data) it is unrealistic to expect that there will be only one, consistent install experience for everything.
In other words, installing the plugin agent to monitor your MongoDB will probably always be different from installing the plugin agent to monitor your John Deere tractor.