Maybe it's just me, but I like to know about improvements in the tools I use. I agree that you don't want to force users through a mandatory/self-congratulatory “What's New!” process, but some subtle highlights of the shiniest new tricks can actually enhance a user's experience. I don't want to discover new functionality by accident, especially if it fixes an annoying gap in the system (iPhone cut-n-paste is a great example of this). So yes, be subtle and allow me to opt in to the new features, but let me know that they exist!

The problem with not alienating your customer base with “upgrades”, however, is that the market passes you by. New players can come on the scene with something much better, using the newest technology, right out of the gate, and unless you revolutionize your product, you'll be left in the dust. So, you're stuck with a conundrum–keep your customers dumb and happy, or give them something vastly better but risk losing them due to their realization that better things are possible.