Meh, maybe at scale. I'd say you'd be better off bunging everything in your db at first, and then extracting bits into more specialised data stores based on testing and real-world performance data.
To take one example from there - storing images in your db - this might actually be fine if you have a cache in front of your app servers, since your db only takes the hit once and then nginx/varnish serves the request after that.
Doing replication, versioning and backup for postgresql across a bunch of app servers is fairly straightforward. If all of a sudden you're doing it for postgresql AND mongodb AND lucene - and keeping all those backups in sync with each other - it gets a lot harder, at least without downtime.
Nosql, lucene etc are great tools, but often they're premature optimization. Make it work first using the simplest tools you have, then add complexity to improve performance only where and when required.
To take one example from there - storing images in your db - this might actually be fine if you have a cache in front of your app servers, since your db only takes the hit once and then nginx/varnish serves the request after that.
Doing replication, versioning and backup for postgresql across a bunch of app servers is fairly straightforward. If all of a sudden you're doing it for postgresql AND mongodb AND lucene - and keeping all those backups in sync with each other - it gets a lot harder, at least without downtime.
Nosql, lucene etc are great tools, but often they're premature optimization. Make it work first using the simplest tools you have, then add complexity to improve performance only where and when required.