Honestly, I had to take a look at in depth to respond, since it has been a while since I looked at the google api in depth. What I still notice is that Openlayers and the esri allow for a lot more service types. You have KML and GeoRSS, which are fine, but do not seem to support many other of the more common formats out of the box such as Arcserver services,WMS, or WFS (which are probably the 3 most common types in the GIS world). I have seen some workarounds for these, but they seem hacky and slow in the implementations I have seen. I do not have a big problem with KML, but I cannot always change what format a 3rd party serves a map as.
The other thing that appears to be lacking is front end geoprocessing. With esri at least, you can do quite a bit of geometry manipulation out of the box. This can be extremely useful for realtime data visualizations, as starting up a new map service is often a bottleneck with any of the backend platforms. If you can just render the new layer on the client, you save loads of time.
My last issue might just be a lack of understanding, but from what I have seen of the backend offering that google maps enterprise offers, it is missing some major features. For one, the documentation is lacking, and it appears that the entire rest api is marked as experimental. Second, although it allows for some basic hosting and whatnot (easily replicated by a free server like geoserver, which is trivial to set up), it once again does not have any real processing engine, so you have to download all of the data to another server you own anyway to do any real work on the data.
That being said, I really do think it is a great product, and I definitely point people in your direction if they are looking for a quick an easy way to add a map to a site with some basic data. At the same time, I am not sure the enterprise plan really makes much sense, since the results can be so easily replicated with geoserver + openlayers and you already need a server solution anyway for running a processing engine like grass or pyqgis.
Also note: I am probably not your typical use case, and I realize that most people just need to throw preprocessed layers on a static map. All I can say is that I look forward to the day when I do not feel like I am the only one around doing dynamic big data GIS on the web, so I don't have to keep building all of this stuff from scratch ;)
I also want to mention that I retract the part about leaflet. I think you guys have wayyy more features than them, but they are the new kids on the block so we can cut them some slack. Also, their controls are the best designed for aesthetics and usability IMHO, which is no small matter.
