I understand that the limited amount of data can make for some easy false positives on trading algorithms, but how relevant do you really think market movements from over 11 years ago really apply to today?
The one elephant that jumps to mind is liquidity. The mechanisms by which liquidity in the markets affect the end results are absolutely key to how your algorithm's interactions with the data will pan out. With that in mind, the methods by which we come by liquidity have dramatically transformed over the years, and market makers from >10 years ago operated in an entirely different way. That's my $0.02, anyhow.
Anecdotal evidence: I have first-hard experience implementing algorithms that chugged along just fine with 10 years of data for backtesting.
You could ask the same question about many different regulatory changes, but Reg NMS is likely to be the biggest.