Well,
1. Hillary has higher unfavourables than most other candidates barring Trump and Cruz. Sanders is the only candidate across both parties with positive ratings. Favourables have been a reliable indicator of the winner of the presidential election for 30+ years.
2. Sanders does substantially better than Hillary, in both polling numbers and favourables, among independents.
3. Every single general election matchup (Sanders v Trump, Clinton v Trump, S v Rubio, C v Rubio, S v Cruz, etc) has Sanders outperforming Hillary. He beats Trump by bigger margins than she does, and he beats Cruz while she is trailing to that obnoxious piece of shit.
Now, this argument has gone on for many pages, and all of Raoul's and my points can be refuted, but there are 2 sides to this. And the current polling data, rather than gut feel, is on Sanders' side.