I have just upgraded this WordPress installation to the new WordPress 4.0 platform. So far everything seems fine. I know in the past the Thesis theme that I use has had some conficts with the newer WordPress updates, but usually it gets sorted out pretty quick by Chris Pearson and his team.
I am enjoying how it is getting much easier to upload images into WordPress posts and pages now and the new media library works very well. I can imagine that in a year or so time we will be able to actually just edit right on the page, which will be very nice. But for now WordPress remains to be a major part of the internet and it looks like it is here to stay for a long time.