I don't necessarily think Gatekeeper is a bad thing, but I worry about the impact if Apple were to make the default setting for Gatekeeper be Mac-store-only. Having the option to install other software would be great, but such a move would further marginalize non-Apple-approved software in a huge way. Assuming a large portion of Mac users are not the type to lower default security settings, then that could have a severe impact on the way people think about software in the "wild".