In theory, a big data cluster uses low cost commodity hardware (2 CPUs, 6-12 drives, 32 GB RAM). By clustering many cheap machines, high performance can be achieved at a low cost, along with high reliability due to decentralization.
There is little benefit to running Hapdoop nodes in a virtualized environment (e.g VMWare), since when the node is active (batch processing) it may be pushing RAM and CPU utilization to its limits. This is in contrast to an application or database server which has idle and bursts, but generally has constant utilization at some medium level. What is of greater benefit is a cloud implementation (e.g. Amazon Elastic Cloud) in which one can scale from a few nodes to hundreds or thousands of nodes in real time as the batch cycles through its process.
Unlike a traditional n-tier architecture, Hadoop combines compute & storage on the same box. In contrast, an Oracle cluster would typically store its databases on a SAN, and application logic would reside on yet another set of application servers which probably do not utilize their inexpensive internal drives for application specific tasks.
A Hadoop cluster is linearly scalable, up to 4000 nodes and dozens of petabytes of data.
In a traditional db cluster (such as Oracle RAC), the architecture of the cluster should be designed with knowledge of the schema and volume (input and retrieval) of the data. WIth Hadoop, scalability is, at worst, linear. Using a cloud architecture, additional Hadoop nodes can be provisioned on the fly as node utilization increases or decreases.