Limitation - they have either a CD-ROM or a DVD-ROM, no burner, and the slot load optical drives are neither easy nor cheap to upgrade.
They will however take up to 1 Gb of PC100/133 DIMMs, and that is easy to install. The DV models would be more desirable because they have Firewire. The non-DV models with USB1.1 only are a drag to attach mass storage to. The 15" screen is decent enough.
I would say a 400 MHz iMac DV with 1 Gb RAM and either Jaguar or Panther would be quite viable for many tasks. Certainly I have done Photoshop and Quark professionally on much weaker machines. It will run most programs, including audio and video editing, just slower than the current lineup.
Remember too, you don't necessarily have to use the latest version of a program to get the job done.