I actually watch streamed video games quite a bit. There's this guy who started doing it a year and half ago. He was on hiatus at one point, but other than that he's pretty much streamed nightly at midnight Eastern time.
www.kuoushi.com
Some of his regulars started taking interest in streaming themselves, including me, but I'm pretty busy with school at the moment and almost never have time for it. With the current block there are 4 streamers who stream back to back starting at 11 eastern time. I throw it on and leave it running in the background most of the time, and check in every now and then. I can tell you what I personally like watching.
RPGs are pretty boring to watch for the most part. You, as the streamer get to read chat more often and be more talkative. You can use conversation skills to make this worth watching in between interesting bits.
There are some games that will make a stream worth watching all the way through, something unique and controversial, like Candy Van.
After that I think, it just depends on the game and the streamer. There's a streamer on kuoushi.com who just attempted a nuzlocke run through Pokemon (Hardcore mode, if a pokemon faints you release it. You can only catch the first pokemon you see in a new zone). He also nicknamed his Pokemon after people in chat. Personally I think that was a pretty engaging stream. Even if you didn't catch it that night, you'd wonder if he had got the Pokemon named after you killed.
Here's a 5 minute stream I did of Metal Slug while someone else was running late:
http://www.livestream.com/lasercakes/video?clipId=pla_f04bb601-c1d4-4893-86df-4a7afc8ea4bb. I also attempted to stream Touhou which can visually be very busy at times and would lag the stream down to 4 frames per second. It was also nearly impossible to read chat sometimes. I am currently playing through Doom III which is actually a quick-save/quick-load-fest. I really couldn't tell you if people like watching it.