What has been the most significant development in #marketstructure over the past decade, and why?

We put the question out to our 11,200 Twitter followers. A sampling of responses:

The more things change, the more they stay the same (avg b/a spreads <2 cents different vs 6 yrs ago) #optionsliquiditymatrix

— Hanweck (@HanweckRealTime) December 1, 2017

Investors have become so much more educated as to how a web of complexity was used to purposely create edge for fast traders at investor expense. Courtesy of exchanges and crappy dark pools.

— Sal Arnuk (@ThemisSal) December 1, 2017

It's just under the wire, but I would say the go-live of Regulation NMS in early 2007. Am I right? #mm10

— Rob Daly (@rob_daly) December 2, 2017

The NMS trade-through rule was and is unnecessary in a world with best-ex requirements. It merely enshrined what technology was already doing. MIFID2 will have a much larger impact as it affects not only market data and competition but also research.

— James J. Angel (@GuFinProf) December 4, 2017

@marketsmedia, I think technological complexity has impacted #marketstructure in the past 10 years. Complexity may have increased, but more dramatically, our ability to live with it has decreased. #cloud will enable us to reduce this complexity significantly

— Anders Kirkeby (@akirkeby) December 4, 2017

A massive shift in profitability from brokers to the exchanges; exchanges provide data, analytics, tech, & both trading & issuer services, while broker payrolls have shrunk & profits decline. Initially, this cut trading costs, but that has stopped as infrastructure costs mount

— David Weisberger (@daveweisberger) December 4, 2017

So much to choose from…but I think the evolution and growth of non-bank liquidity providers has had the biggest impact on the market overall.

— Kevin McPartland (@kmcpartland) December 4, 2017

Post crisis, the most significant development was turning the once opaque #OTCderivatives market into one of the most transparent markets in the world as a result of #tradereporting requirements & the introduction of infrastructure to support mandates. #mm10 https://t.co/Ol9FMAXhih

— DTCC (@The_DTCC) December 4, 2017

Data – from direct feeds, SIP data, FIX data, pre trade data, port trade data, liquidity heat-mapping, alternative data… equity markets at least have become heavily data driven.

— Richard Johnson (@_richjohnson) December 4, 2017

What we believe is the most significant #marketstructure development in the last 10 years is that exchanges now have a fiduciary responsibility to shareholders that has affected policy with respect to other market participants. #mm10

— Deep Systems (@DeepSystemsHQ) December 4, 2017

Explosion in ETFs #mm10

— Chris Ekonomidis (@chriseko) December 5, 2017

10 years ago, the question was, “How does technology fit into my workflow.” Now, the question is, “How does my workflow fit into technology?” Technology has taken the ambiguity out of the investment process. – Adam Steinhaus, Portfolio Management & Trading Solutions

— FactSet (@FactSet) December 5, 2017