Found out myself:
Window - Preferences - Java - Editor - Content Assist - Advance
Check Java Type Proposals.
However it is not very nice to distribute an Eclipse Version like this. I guess many user will not immediately suspect that such a basic feature has been disabled by configuration.
Sorry for bumping into what looks like a bug.
Any chance you used an old workspace that had this setting disabled ?
There was actually a bug in older Eclipse version that would tricker disablement of this java type proposals but I haven't seen that in ages and should not be present in JBDS 6.1.
If you did use a new workspace and can reproduce this error please let us know the exact steps you did.