1. Signing should be different from source integrity. For example, signing the manifest that contains a bunch of hashes should be sufficient - you don't necessarily need to sign the entire compressed package. Newer package management systems like the Illumos IPS work this way (which is not without it's faults). The veneration of the "church of tarball" needs to go away - we've been living in a DVCS world for the past few years and it's awesome compared to the dark ages of before.
2. We need some sort of heuristics to help make decisions. For example - a diff between versions is much easier to audit, and having statistics on contributors might be quite useful. For example, red flags might go up if a previously slow moving project had a massive number of commits with a new developer - this might just be new blood getting into a project, or a maliciously intended merge.
In the end, eyeballs are needed. We need to make it easier for those eyeballs to go further on less.