Not being able to see what types code deals in, jump to definition, and find usages makes me feel crippled when exploring a new codebase.
One of my wished for programmers everywhere is that tools like Github and BitBucket start analyzing projects and letting you navigate better. I think that could save thousands of engineer hours.
This topic alone is a huge reason I started using Dart and eventually joined the team. Trying to figure out how a very large JavaScript codebase works is so incredibly painful, doing so in Dart is incredibly easy. This is also where very reflective libraries like Guice go wrong, and why, in my opinion, meta-programming should be used very carefully and sparingly.