Drives are definitely the answer. My miniDV setup worked fine with 5400rpm, but HD playback looked like internet video from 2004. Upgraded to 7200rpm internal SATA drives, and ~most~ of the time, I've got good results. While I've never done any work with SSDs, I suspect those would be the ideal circumstance.
For external drives, I've used 5400rpm USB2.0 and FireWire (ieee1394) with DV just fine, but even with external drives, there may be bus speed issues on top of drive speed issues.
One thing that I have had good luck with is DSLR footage. I shoot 1080i on CF cards and 720p on SD Cards. I've got an internal SATA card reader, and I've been very impressed with throughput directly from the cards.
I've found that most editing lag these days is more related to drive speed than anything else. Most people doing video editing are savvy enough to at least know to ask "Will video editing need a more robust machine?"
RAID 10 is something I'm exploring right now. It supposedly combines striping (for faster access) with mirroring (for reliability) on a 4-disk array. Some IT professional friends of mine say that it's stable enough that they're putting the OS on the same physical drives as the data. An interesting thing to consider.
Hope this helps...or if nothing else, helps corroborate other information and ideas you're hearing.