i like the new modelling features, especially because i've been waiting so long for some (free) support in that area in eclipse. all the uml plugins i've tried so far were either quite limited in their functionality or their usability. but now all sorts of uml diagrams are supported, e.g. class diagrams, activity diagrams, use case diagrams, etc. What's missing are some kind of reverse engineering features and sequence diagrams (or i haven't found them yet).
the new breadcrumbs feature shows the path to the current item at the top of the editor. it's like
project -> src folder -> package -> class -> class member. each of these crumbs has a context menu which shows other items at the chosen level. i think that's quite nice, as it allows more context-sensitive working like mylyn task focussing, which makes it easier to find items than in the package/project explorer.
one thing i'm still waiting for is a mylyn feature that allows for creating reports, e.g. a report that lists all completed tasks per week with estimated time and actual time. sadly, mylyn hasn't been improved that much, i think. but still it's a great feature.
these are just some of the new features and improvements. there are many smaller ones, like junit test excectution time, enhanced tooltips (e.g. javadoc on types), autocompletion, subversion support, etc.
one feature i haven't tried out so far is the ECF (eclipse communication framework), but that is one i'll try out for sure. from what i've read, it seems to support remote team collaboration, e.g. to share an editor. that sounds interesting.