Although at first glance there seem to be few differences between 2010 and the older versions (I skipped 2009 so I don't know how it compares.) but there are many little things I am finding in which 2010 shines above it predecessors.
There are also the not so little things that make a huge difference, like draw distance, performance increases in native mode, layers in surveyor. All of which are huge improvements over previous versions, 2006, TC3, etc....
Some have criticized the franchise in general for re-using a lot of things, the engine in particular, but for all that Trainz 2010 feels more solid, more polished than any previous version I've used.
Granted, there are always growing pains, speed trees is a good example. Right now I am torn, generally they look much worse, more "cartoony" than the many high quality billboard trees available so I avoid them. But some of them look quite good, and the fact that they are fully 3D, react to seasonal changes, etc... makes the possibilities far greater than the old style trees. Give the content creation community some time, they will no doubt eventually eclipse the old trees easily and become the new standard.
Overall Trainz 2010 feels more polished, than ever before. In the higher quality native routes, like the York to Kings Cross HST, or high quality DLS routes like the Clovis Sub updated to full native mode compatibility, Trainz 2010 looks every bit as good as Rail Simulator, and still has all the features which set it apart from it's competitors like industry and operational aspects.
It is winning the community over, I think, deservedly so. Check the screenshots thread, there are some amazingly good looking routes in development for 2010.