Hi!
As an HD Only customer myself, I can tell you that you get ALL the non-premium channels (HBO, Showtime, etc). The list on the website is *mostly* correct, but you don't get Big 10 unless you are in one of the states that is supposed to get it, and you *do* get SciFi HD and USA HD even though they aren't on the list for some reason. In addition to the HD versions of the stations, you do also get the corresponding SD versions of the stations.
I'm not sure why they didn't list Sci-Fi and USA to the HD Only listings on the web page -- my worry is that, eventually, Dish Network will come back and claim that HD Only customers got SciFi HD and USA HD as a "free preview" and that we have to subscribe to the "America's Top" packages to get them. This Kinda makes sense to me since, most of the time, networks make all subscribers get ALL the stations the network owns if a provider wants to carry the "big station" -- IE, if a provider wants to carry The Disney channel, they must also carry Toon Disney, ABC Family, ESPN, ESPN2, ESPN News, etc. I honestly think this is why Dish is having problems getting contracts signed to carry HD stations -- Dish doesn't want to give customers SD stations with the package, but the providers want Dish to give them these "secondary" stations if they are going to carry an HD station.
If you subscribe to locals, you do get your HD locals and SD locals. So, if there are no HD locals, you would get SD locals. You also get NASA TV and a bunch of SD shopping channels with the package.
Hope that helps!