You're now at the point where the easy desicions have been made and where most people get brain freeze trying to pick a NAS. The main hangup is usually described by newbies looking for buying advice as "not wanting to pay for features that I won't use/ don't need". This is usually followed by a laundry list of features that they would like to have.
Fortunately, NAS makers have made this choice easier by adopting common feature sets across their product lines and expanding them to cover a wide range of needs (or desires). So in most cases, whether you choose the cheapest entry-level single drive or eight-bay quad core powerhouse, you'll get the same features. So what features are you looking for?
General File Storage & Serving
As we said at the start, all NASes provide central file storage and access to pretty much any device on your network.
These duties can be supported by any NAS if you don't need UPnP / DLNA or iTunes serving. Performance of even entry-level NASes has risen to the point where the 20 to 40 Mbps (2.5 to 5 MB/s) requirements of full 1080p media streams can easily be met for even multiple streams (see Can Your NAS Do Two Things At Once?) . And there are plenty of entry-level NASes that throw in a basic UPnP / DLNA server and usually an iTunes server, too.
If you want media streaming to mobile devices, web-based photo viewers / slideshows, on-the-fly transcoding or anything else special in the media serving department you'll need to look more carefully at product specs. Synology and QNAP probably have the strongest offerings in this area, with Synology currently having the edge (see Deciding Between Synology & QNAP - Take 2 for details).
But also take a look at ASUSTOR, which seems to be catching up fast. They, along with QNAP, have HDMI ports on many of their products, allowing them to be used as media center / HTPC replacements. Synology doesn't believe in the concept of NAS as media player, so have nothing with HDMI built in.
Backup is another core NAS function where things have changed a lot in the past few years. Many NASes now support backup and file sync on local networks, secure remote connections and cloud services such as AWS, Dropbox, Google Drive and more. "Private cloud" features that provide easy secure remote access to NAS files have also become more common, as has support for Apple Time Machine backup.
As noted at the start, because all NASes support shared network folders, they can be used by any client backup app / utility running locally. If you require backup or sync to remote NASes or cloud services, then you'll need to dig into product specs.
NETGEAR ReadyNASes continue to have very flexible backup options for both client and NAS backup. They perform immediate and scheduled backups to and from most anything (FTP, SMB and NFS shares) and have reliable emailed backup reporting. ReadyNASes are the only NASes to support unlimited volume snapshots that allow rolling back the NAS to any saved point, the only to have "bitrot" protection and the only to use the Btrfs file system.
Seagate's latest NASes (post LaCie merger) also can back up to / from rsync, other Seagate NASes, FTP servers and SMB, NFS and WebDAV shares.
If you don't want to spend a lot on a NAS and easy and reliable remote access is important, consider WD's MyCloud. It comes in No RAID, RAID 1 and RAID 5 flavors, doesn't require ports opened in your router's firewall and works through multiple NATs (which you have if you have a combination modem/router that can't be bridged and have added your own router).
In general, when looking at remote access or "personal cloud" features, be sure to find out if remote access requires ports to be opened in your router. QNAP's myQNAPcloud, for example, requires opened ports, but will attempt to do it via UPnP. But this doesn't help if you have multiple NATs or if you choose to disable UPnP for security reasons.
Another thing to explore carefully if you will depend on it is rsync support. Most NASes now support this network backup standard to and from generic rsync servers in addition to their own kind. But, for example, Buffalo NASes continue to support network backup only to other Buffalo NASes.
Another feature that has become mainstream since our last look is the ability to expand NAS features via installable apps. While this is attractive to future-proofers, tread carefully. Companies like to boast of how many apps are in their libraries, but many are from third parties and not supported by the manufacturer. If they don't work, you'll be relying on user Forums and your favorite search engine to get them working.
Again, if a particular feature is important to you, see if it comes in the default OS install or is officially supported. For example, QNAP flags supported apps as QNAP Select in its App Center for easy identification.
Once you get beyond the feature sets above, you're into more specialized requirements. Since high-end NASes have gotten powerful enough to push aside servers for some uses, many are being used for virtualization. Many NAS OSes come with VMware Ready, Citrix Ready, or Microsoft Hyper-V compatibility promises or certifications. But QNAP has gone a step further with its Virtualization Station feature, which makes it easy to configure VMs and load saved images.
If data security is important, look for products that support built-in volume encryption. Synology's DS415+, for example, has a processor with hardware encryption that can reduce encryption overhead to zero in some cases. Built-in anti-virus is also available on Thecus, NETGEAR, Synology, QNAP and ASUSTOR NASes, among others.
Finally, if you need a lot of storage, look for iSCSI and SAN features. Both Synology and QNAP have models that use eSATA or Infiniband-connected expansion cabinets to add more drives. But QNAP also has a built-in iSCSI initiator that is needed to build SANs. QNAP also recently re-engineered its storage manager to add Storage Pools for building high-capacity storage systems.
When it comes down to the final selection, use our NAS Ranker for quick picks. It combines rankings of multiple benchmarks into an overall score for easy comparison. You can also view sub-rankings for Read, Write, Mixed Read/Write, Video, Backup and iSCSI performance.
When researching feaures, hit the NAS Finder. It lets you narrow your search with a multitude of filters for a wide range of product attributes.
And, finally, don't forget the NAS Charts if you want to compare products benchmark-by-benchmark. Happy hunting!