What data structure does your database use? It's not something the typical database user spends much time pondering. Since data structure is destiny, what data structure your database uses is a key point to consider in your selection process.
We know CouchDB uses a modified B+ tree. We've learned a lot fascinating details over the years about the use of Log-structured merge-trees in Cassandra, HBase and LevelDB. So B+ trees and LSMs seem familiar by now.
What may not be so familiar is Tokutek's Fractal Tree Indexing technology that is supposed to be even better than B+ trees and LSMs.
As a comparison between Fractal Tree Indexing and LSMs, Bradley Kuszmaul, Chief Architect at Tokutek, has written a detailed paper, a must read for the algorithmically inclined or someone interested in database internals: A Comparison of Log-Structured Merge (LSM) and Fractal Tree Indexing.
Here's a quick intro to Fractal Tree (FT) indexes: