The part I like about the HTPC so much, is the flexibility. Not only can I view the sources listed above, but I can stream from the more specialized websites. For instance, I like watching cooking shows on foodnetwork.com. I'll watch shows off HGTV.com. It's hit or miss with these shows on Hulu or Streampix. I also like that with free software, I can easily have a VERY NICE front end to my 350+ DVD collection on my basement server (MediaBrowser 3). I can also play a real game on it fairly easily. Just nothing too graphics intensive, (It plays Civ5 pretty well. It's an i3 processor with a cheap passively cooled video card I think I paid $10 for after rebate from NewEgg). I also just love having the web browser on the TV. An example would be when my wife and I go over our finances, I can pull up the bank website on my TV and go over our finances for the coming month. It's a lot easier than trying to both look at a laptop or a small screen.
Having said all that, while I love my HTPC and I'd pay another $500 to replace it today if it broke, I would never recommend it to anyone who doesn't have a reasonible technical aptitude. There is certainly something nice about having a small box that you plug in, enter your account details, and it instantly connects you to your movies/shows. If it breaks, it's just $60 for a new one, so who cares, right? That's what end users want. It's much less complicated. My grandma can use a Roku, but she can't even use the DVR on her cable box. It's all about simplicity of design and ease of function.