continuing from a comment in the design thread...
everything is peachy until you actually start making real data. Real data = millions of polygons. For example Jak and Daxtor's levels are millions of polys as are the levels of Unreal 3.
What that means in terms of Collada are things like using an ascii based format for vertex positions, UVs, colors, weights and normals may not be a good idea. Maybe a well defined CDATA format would be better in those cases.
It also can mean issues for XML parsers and the design of the format. Since most XML parses store lots of extra info PER internal element, any schema which ends up with thousands or tens of thousands of elements should be avoided. An simple example would be if every vertex was a separate element. That's not the case with Collada so far.