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1. All the UI code becomes open source. This is not desirable and it gets worse as more of the code moves to browser in order to build RIA apps.
2. UI code is monolithic. No components, not even namespace or classes.
3. No first class IDE support. Very poor productivity.
4. Difficult to debug (or understand esp. if someone else wrote it).
5. Inconsistent behavior across the various platform/browser combinations.
7. Security problems. It will only get worse as you consume untrusted services from browser.