You can run Opera without Java if you just want to try it out. Java is used by some websites. It is a kind of scripting language for doing fancy tricks. You will lose some functions if you don't have it installed, but will still be able to browse normally.
Opera has no security problems at all. If you browse heavily and have several pages open at once its multiple page tabs make it easy to switch pages. The interface is extremely customisable which also greatly speeds up browsing. If you don't flick back and forth much, you won't see any great improvement, apart from a prettier face. Opera has a wide choice of skins.
If you are short-sighted, you will see an enormous benefit, as Opera can zoom in and out with the + - numeric keys. Some web pages like this one have fixed font sizes.