I guess it kinda says itself, that running an additional VM will slow down any process, as you only can work within the bounds of that. It's both it's strongest and weakest side.
Another one of the stronger sides, is that it's really easy to learn, but hard to master. As mentioned before in this post, a real pro can make java work almost as fast as other languages, but that requires a lot of work.
I don't know why, but OOP comes very natural to me, so I think that's great. Not that there isn't other languages that does that.