I've only used it for a few hours but here are the first things I found:
1) Default Java version is Java 1.6, and OmegaT seems to have issues with that. It works but there is an issue with the font selection that is not respected in the Editor window.
2) The input method can be set to stick to a document. For ex, you can have a TextEdit document that uses Kotoeri, another that uses US and changing of document will not change the input method. That's an improvement over the situation in Tiger where, if I remember well, the input method was "attached" to the application but not to the document.
3) The old option "multilingual spellchecking" can use a subset of the available languages. That way, you can more efficiently check the spelling without changing the language in the spellchecker interface every time a paragraph uses a different language.
4) The spellchecker can also automatically correct your mistakes.
5) The services are indeed much better integrated to the workflow. A right click on an element (text, image etc) will trigger a contextual menu that shows possible services for that element.