This top five list wasn't the hardest I've ever had to write for an application upgrade, partly because the list of new features in After Effects CS5 is unusually short, with 64-bit conversion having dominated the development team's resources. But even if the new additions are few in number, they make a big difference. That 64-bit pipeline alone justifies the cost of upgrade for any studio doing heavy-duty work, because workarounds for memory limitation are time-consumingand even, in some cases, deal-killing.
The only extra downside at this writing is that all After Effects plug-ins will have to be upgraded for 64-bit compatibility. That non-trivial assignment ensures that not all of them will be ready immediately. For those plug-ins that are ready, some manufacturers will require an upgrade fee to cover their own development costs.
Any studio that never got around to upgrading from CS3 to CS4 now has a solid set of reasons to move forward to CS5or be left with an increasingly antiquated set of tools. Once you've had the experience of keying Red R3D files natively in 4K or creating whip pans around huge matte paintings, you'll never look back, and the other After Effects features that go beyond memory management are sweet icing on that cake.