Steam Machine: Providing Games without the cost of hardware updating common on the PC platform. Basically utilizing the 'server does the work' approach, where the servers (like youtube) process the gaming, the graphics, etc. and 'stream' the video results to the Steam Machine to display, the steam machine then reads your inputs (move left, look right, fire weapon) and sends them to the server to process. This concept succeeds only on HIGH bandwidth connections.
PS4: Console that will also 'STREAM' gaming then instead of downloading to internal HD and loading locally on the PS4 to then connect 'online' to play. While DVD and downloaded gaming still occurs, the console provides the EXACT same experiance no matter who plays it, when it is played, what game is loaded, what TV, controller, etc. is used. As compared to PCs this is a godsend, as you never have to worry about viruses, drivers, if your power is enough to power the graphics, a new game coming out 'won't play' because your system doesn't meet the demands, etc. All that is not a issue and there is no 'management' (updates, patches etc.). For EASE of solution there is none better answer, except MAYbe the Steam box (best of both worlds?).
PC: Costly, high maintence, prone to 'issues' that frustrate those without technical skill, complex and 'your on your own'. For all this you get the ability to mold the system to your tastes and needs, you can use the computer for any purpose you see fit / want to spend on. Steambox/Console is a SINGLE PURPOSE device, you can't expect it to scan in your W-2s to email off for your tax filing on them, but you can on a PC. Want better graphics then your neighbor, or maybe now you want to have a three screen display, sure buy a more powerful PSU and GPU and boom now your rocking. Want better performance in your gaming, Keyboard / mouse ALWAYS beats controller in FPS. And so on, but you need to be honest with yourself, there is no 'cheap' model for PC Gaming, and you can't 'short cut' your way. Think of it as your body, if you 'cut off a leg' just because it costs too much, can you really expect to perform in the top 10 of the Trimarathon? If you don't keep yourself healthy, do you honestly expect never to get sick? Same for computers, don't keep up on maintaining them (cleaning them out, running virus and malware scans, etc.) then they get sick and break down too (just think how bad you felt with the flu, 10000x worse for the poor PC).