What advantages does in-memory OLAP have over traditional systems with significant memory?
Not my field of expertise but as I understand it the difference in the majority of so-called in-memory OLAP databases (not a term I'm fond of, it's used as marketing pitch more than as a fair comparison of technologies) is column store indexes.

Column-Stores vs Row-Stores (How Different Are They Really) is a good intro to the technology if you're familiar with traditional OLTP and OLAP database structures.