Leaflet allows you to do the basics, of course, such as popup an information window and overlay your own data. It can do far more than that and is a viable replacement for Google Maps. The functionality brought by Jason Sanford’s Leaflet Vector Layers is a real selling point for me. Geocommons are ArcGIS Online are two easy ways for people to get their data on the web. Leaflet Vector Layers then makes pulling that data into a web map simple. The Vector Layers plugin also makes Arc2Earth data available to you. Arc2Earth is a great tool for helping you move your data off of the desktop on to a cloud-based service. They have been doing some amazing work with storing geospatial data in Google Fusion Tables. However you share your data on the web, Leaflet can bring it in to a web map and display it. If it’s easy to code and produces something that is easy to use and aesthetically pleasing, it’s going to be a big success.
I am working on a project at Rowan where we are looking to make an interactive campus map that will zoom down to the room level, displaying floor plans of the buildings on campus. The flexibility and the cross-platform functionality of Leaflet means that I will seriously consider using this Library to power the user interface of the map. I was originally thinking about OpenLayers, but Leaflet seems powerful enough to do all that I require.