There are many definitions for cloud computing around, and depending on the time of day I'd probably recite a different one. But what's really defining to me is the availability of quasi endless computers (instances in EC2 terminology). Pricing model, pay as you go, someone else deals with muck - all great stuff. But endless computers, that changes computing. Why? Because launching a new instance becomes the one cure to many problems.
Whatever happens that is uncool, the answer becomes: launch a fresh instance! (Or two while you're at it.) It may be an instance failing, a hard drive starting to give out, or excessive memory errors. It may be the load going up and about to max out the existing servers you have. or installation of a new version of your application that you'd rather bring up and test before cutting production over. It may be the network not behaving cleanly because of some failures or other issues, or your latest app upgrade causing a slowdown because you overlooked a slow database query. In all these cases, if you've prepared the ability to launch a new instance quickly and cheaply (labor-wise), you're out ahead. That's cloud computing!
So get comfortable and set up your tools to make the launching easy. Everything else follows. This is of course what we're enabling at RightScale, but even if you decide not to use RightScale it's the way to go. Launch!