This was not about you or anyone specifically playing around with the Stream Mini, but it has major shortcomings.
If someone is considering one of these, then there is a post by someone that considered buying one, but went with a different option that gives similar performance at a similar price. (after the RAM and SSD upgrades added to the Mini)
A different perspective is good to have.
It is good to compare other hardware. I can also see some cons to this:
1. Still a good deal more money to build it yourself - the barebones you priced out was $181
without OS, and not everyone has Dreamspark to get a copy of 8.1 or can find as good of a deal as your Windows 7 price from a legitimate online retailer (I have no computers stores in my area - CompUSA & all the others dried up, I have to order 100% of my stuff online these days) And if we really want to do a 1:1 comparison, you'd need a full copy of Windows 8.1 ($99) & new hardware (not used), which increases the price even more.
2. The motherboard you mentioned was used. While that is a good cost savings, it's not the same as having a new, fresh-from-the-factory, warrantied board like you would with a new Stream Mini system, especially if you're putting it into a work environment or into something like a grandparent's PC that you want to run reliably for years & get factory support for going into the future.
3. It's awfully nice having a turnkey system that is also budget-friendly. I have a customer who is interested in these units as a budget system; they have been running on the same Dell desktops with Windows XP for a good 10 years straight now & don't have $50k to invest in a nicer NUC/BRIX system with new monitors. For around $300, I can buy a Stream Mini, slap in a stick of RAM, & upgrade to 8.1 Pro to join it to their domain and have a significantly improved end-user experience that gives them modern compatibility. Times 30 units, that's under $10k for a nice upgrade for their facility. For me, the installation process is basically unscrew the bottom plate, snap in the extra RAM, boot it up & paste in the Pro Pack key, then join it to their domain & setup the user. Really really easy process with no major assembly required.
4. I like having an integrated system from the factory. I've built a variety of ITX barebones like the RAIDMAX you linked. You have to make sure every piece works yourself, plus ITX boxes can be iffy sometimes (cooling, tiny fans, etc.). With the HP Stream Mini, you can literally take it out of the box & have a working system in minutes after plugging it all in together. So that's a nice benefit if you don't have a lot of time to test everything yourself.
So while building an equivalent mini system is not without merit, it's not exactly a 1:1 comparison either. Also, thanks to cloud & NAS storage, having a small local hard drive isn't as big of a deal anymore, and 2GB RAM is surprisingly efficient under Windows 8.1, so it's decently usable for a non-power home user out of the box.
I build a lot of systems based off the NUC platform (mostly Gigabyte BRIX 4" x 4" computers at my main job) & they are really great (fast to build & I can restore off a master image in about 15 minutes), but the price adds up super super quick. Most of the units I build (256gb SSD with Windows 7) clock in at about a grand a box with a copy of Office. I can do the same thing with a Stream Mini for literally half the cost.
You're right though, the price tag is deceiving if you really want to use it as a decent computer - at the very least, you'll want to add at least a $30 stick of RAM to boost it up to 6 gigs total, which pushes it over the $200 mark (still pretty decent imo). But the CPU is locked in & you can't upgrade the GPU at all, so your options are pretty much limited to SSD, RAM, and OS upgrades - but no matter how much hardware & money you dump into it, it will still be a Celeron box with integrated graphics at the end of the day, with no path to upgrade those.