The reason why I am skeptical of new entrants into the javascript package management arena is that there is already a package manager with over 14k packages and a vibrant community that is producing packages at a very high rate: npm. npm might not be the best package manager as-is for browser development, but tools like browserify (disclaimer: I wrote browserify), ender, or many others can let your browser code use much of the value that has already been created for npm, even though npm is primarily about node.js packages. A surprising number of modules written for node will just work in tools like browserify.
Any upstart package systems would be wise to have a clear and simple path for unlocking the value created in the node community because there is too much value being created in npm to ignore. Whether that path involves adding fields to the package.json, tools to bridge the gap between package repositories, fancy in-browser trickery or compile steps remains an open question but it would be really pointless to require people to publish their modules that they've already put on npm to buy in to yet another package system, especially since there seems to be a new package manager every month lately.