I'm the kind of person who loves to think alternatively how games could have been when they were being developed. There are MANY games out there with content never shown to the public, extremely different stories, etc. and I really love all of it. I always found Nintendo and Valve games to be the most interesting during development. I got into betas when I saw videos on Gamecube games. I grew up with the Gamecube (was my first console) so seeing these drastic changes was pretty cool to think about. Some of the most interesting betas I've seen so far (not only Nintendo games) are Luigi's Mansion, Pikmin 1-3, Super Mario Sunshine, Team Fortress 2, and Half-Life 2. What are some interesting betas you've seen?
Yeah, newer games just don't seem very fleshed out during their beta stage. It might just be because of the sense of mystery you get watching low-quality early 2000's or 90's videos on the beta footage of a game from that era.