1. Test automation cannot replace manual testing.
2. Unrealistic expectations.
3. Maintenance of automated tests is quite different from manual tests and some times cumbersome.
4. Writing of automated tests is more difficult than manual tests and requires a superset of knowledge and experience over manual testing.
5. It pays off late.
6. Greater reliance on the quality of the tests.
Source:
stickyminds
sqaforums
Auto_Zhen
testingeducation
Software Test Automation by MARK FEWSTER, DOROTHY GRAHAM