In my view, the android operating system suffers from a few drawbacks as a phone OS.

1. Browser. Bit of a big one this, given how the phone is so heavily marketed for its internet connectivity. It doesn't appear to have very clever use of cache, so that pages sometimes are reloaded when there should be no need (e.g. after minimising),
2. Autocompletion. When typing emails, texts, or filling in online forms using the on-screen keyboard, the list of suggested words is not nearly as useful as it could be. It doesn't learn new words which you've used previously, nor does it favour choices you made previously. Additionally, the words it suggests are not very well arranged. The most common words should appear earlier in the list, while it seems that some arbitary ordering is used, such as alphabetic.
3. Messages. It's not possible to save a draft message without entering a receipient. Nor does the time for a message appear on the main message listing except when the message was received today, which is not very clever. You can't easily clean up messages, for instance deleting all messages over a certain age.
4. You can't easily switch the phone to vibrate or silent, nor can you easily turn on/off GPS, WiFi etc. This problem is adequately resolved by installing the free Toggle Settings app from the MarketPlace however.