I think we need an FAQ or 2 on this!
compborder, that's nearly right - RPM is a large factor in determining how well a disk performs. Check out Seagates' Cheetah drives, at 15,000 RPM, with a potential transfer rate of 160Mb/sec.
The biggest factor, however, is access time. The quicker a disk drive can access sectors on a platter, the faster, overall, the disk will perform. Hence it's advisable to choose a disk with 8m/s access time over a disk with 9m/s access time.
This can be aided by a large cache - some manufacturers are including caches up to 8Mb in the drive's electronics, while caching SCSI boards have been including a DIMM slot for some time. A cache, however, provides virtual access points, so can produce misleading figures in benchmarks. This said, a large cache is a very good thing.
The MB/sec quoted by most manufaturers is misleading, and refers to the drives "burst" rate - the fastest it has been timed at, usually reading data from the inside sectors.
More significant is the STR, or Sustained Transfer Rate. Good hardware sites, such as Tomshardware.com will post graphs so that you can get a good comparison.