A High Frequency Trader's Apologia, Part 2

Nice intro, though I think you can take it further.

One addition I'd be interested to see is how do modern markets with HFT compare to markets 20 years ago, or whenever the pre-HFT era was.

How does the liquidity of each, and more importantly the cost of liquidity of each, compare? Was lack of liquidity really a problem in the pre-HFT era?