We have a semi-corporate environment with about 25 users. Most machines are Windows7 but there are some Macs on the network, too.
We want to set up some location (on a Windows server with lots of storage) where users can store their usual daily working files. This way, we can back up the entire storage drive and not have to run backups on each machine. The idea is that users will keep their important files here instead of their local machine.
Now, I've read about NASs but I am not sure that there is any reason why to use a special system rather than just a network file-share. If I'm not mistaken, even the Macs can access the windows file shares.
A NAS server would add another level of (unwanted) authentication. I am rather inclined to use just Windows file sharing. I don't need a media server.
Am I missing anything here?
For some background:
The reason you typically get a NAS appliance is to get something that will serve up storage and "just work". Of course you need to do an initial configuration, but after that, you pretty much just leave it alone until a drive fails, or there's a critical firmware update. For a Windows file server, you'll need to do regular maintenance (patching, etc).
NAS devices can also offer excellent performance, and nice features like hot-swap/hot-add of drives. They also will handle building your RAID volumes for you with minor effort on your part.
You can join most NAS's to corporate domains, so additional authentication is not an issue.
For your particular scenario:
If you already own the Windows file server (hardware and licensing), then you're pretty much good to go. It should work OK for your needs and there's no reason to replace it with a NAS. My only question would be, does it at least have a RAID1 for the storage drive? I would recommend that in addition to backups ... it will save you trouble in the long run.
HOWEVER - If you don't already own the server, than it is worth looking at SMB level NAS units. You can get a good quality 4-disk NAS for MUCH cheaper than a new server+licensing. ($1000 or so). The only main question here would be if your particular backup solution will work OK if backing up off a remote target (you won't be able to install a backup client directly on the NAS).
You mentioned FreeNAS - that and Openfiler are viable options for your scenario. I have built an Openfiler box used for a similar scenario as yours and it works well. I was able to join it to an AD-based domain and it works fine. If you are comfortable building a rig from scratch this method would be cheap and effective, but keep in mind you're on the hook for hardware repairs.
One thing that (as far as I'm aware) none of the available NAS boxes are capable of (currently) is Shadow Copies which you would be able to do with the Windows server.
This may or may not be important/useful to you; personally I love NAS devices over servers, but just thought I'd throw this into the mix.
Nope, you're not missing anything. If you've already invested in a lot of storage and it can also do NAS - think NetApp or EMC products - then sometimes it makes sense to just use the CIFS functionality built into the storage you've already got. It's one less Windows license you need. You can also leverage storage-based replication and snapshotting more easily than with a Windows server in the middle. Using your storage directly for NAS can simplify DR efforts in some cases. As you've said, though, adding NAS exclusively for what you're asking about is overkill and just one more thing you need to learn.
For an organization of your size, a Windows file server is a no-brainer.