A few observations from what I've seen in the markets...
Significant but unreachable demand for most SPs, in the APAC & South American markets. Infrastructure cost remains too high, without significant hit to margins.
USA - Atlanta, Dallas, Chicago, NYC, Silicon Valley are all key markets... markets like Seattle, Washington DC, Salt Lake City are all growing but not quite the recognition of others.
Europe - London, Amsterdam are primary markets. London seems to be growing in preference, IF you can do it at the same cost as Amsterdam. Unfortunately real estate prices make that hard to do.
APAC - Singapore, Tokyo, Hong Kong all primary markets.
Other things - Miami is always a good way to reach South American market, without nearly the cost. Biggest problem with South America is sourcing hardware into Brazil, where datacenters are probably the most 'mature.' The import tax makes things really difficult. The rise in offsite monitoring, such as wifi enabled pace makers, has caused some markets to grow substantially. Cleveland (represent!) has seen this, with ByteGrid especially taking advantage building out a 300k sq foot datacenter.
Edit: Detroit is also one that is growing significantly. So much cheap real estate, and easy fiber connections have made it a nice option. The city is also going through a bit of a rejuvenation so there's some good tech work going on there.