So here's what's to be excited about for SteamOS.
1. It's game streaming by a company that knows and cares about video games.
Nearly everything that currently sucks about streaming games from your PC to another device is potentially solved by this endeavour. Right now the "brute force" method (which is what companies like Splashtop do) is the only real option we have for PC game streaming: Run a game and copy the framebuffer to RAM, and then have the CPU encode it into video and bounce it across the network. It's a sloppy, clumsy method, and introduces a clearly noticeable delay. It's also usually implemented by people who don't really care about things like latency, framerates, resolutions and image quality. The Wii U/PS4->Vita this is NOT.
The only exception right now is Nvidia's Shield streaming, and aside from being limited to one hardware vendor's PC config, it's also very clearly in BETA. (Shield owner speaking).
What I'm hoping is that Valve takes full advantage of existing dedicated video encoding in Intel (Sand Bridge+), Nvidia (Geforce GTX 600+), and AMD (Radeon HD 7000+) graphics chips, and give us a universal API for streaming to a device using SteamOS with minimal latency and at 60hz when possible. For anyone who's played Off-TV games on the Wii U, this is quite immediately an exciting prospect. Even if they average 2-3 frames latency (vs Wii U's ~1), this would be awesome to have access to.
2. Linux desktop users (both of them)should cheer, because they have Valve in their corner now. The grand majority of optimizations, support, and conformance across hardware vendors will do nothing but improve the desktop Linux experience. Even if their work begins and ends at Big Picture mode, much like for point #1, we benefit from both Nvidia and AMD being forced to offer more than haphazard game support.
[size=78%]3. Devices. Make no bones about it, this is Valve's end game. We are clearly going to have two classes of "Steam Box". The full-on desktop monster running Windows, and the companion/open box running SteamOS. For people who already have next-gen PC powerhouses, having a cheap way to run some games natively and stream the rest will be priceless. We'll see everything from beasts that can run the latest and greatest to tiny Kabini/Bay Trail-equipped $99-199 boxes meant mostly to Stream from beefier machines.[/size]
Bonus: ARM support? I'd love to see Valve make some attempt to keep a partial source of published Linux games on the Steam store to implement ARM builds across titles. A portable or tiny TV (maybe even HDMI plug ala Gamestick and ChromeCast?) SteamOS machine would be amazing. The NVidia Shield's GPU is already 3 times more powerful than the original Xbox; wouldn't it be nice to try the original HL2 or Portal on the go? =)