A knowledgeable correspond tells me:
The [President Leonel] Fernandez administration [in the Dominican Republic] views the midterm elections as particularly crucial given that legislators elected next year will hold a six year term in order to allow for concurrent legislative and presidential elections starting in 2016.
This change will deprive us of an interesting case: since 1996 the DR has been the only country (ever, that I know of) to have exclusively midterm elections: every president has faced an election for the entire (bicameral) legislature at the term’s halfway point (and, of course, vice versa).*
The change will also mean an unusually long term of office. There are few cases of a first or sole chamber being elected for more than five-year terms. In fact, the only other one I can think of is Nicaragua under the constitution enacted during the Sandinista era (and later amended for shorter terms).
When the current electoral cycle was first put in place, the interim arrangement was for a two-year presidential term. The move was in response to the disputed election of 1994, in which the parties agreed to truncate the “reelected” president’s term, while letting the just-elected congress serve its full term.
Now, the Dominican Republic will return to concurrent four-year terms–but not till 2016.
* Others have had exclusively (or usually) non-concurrent elections, as a result of different presidential and legislative term lengths (e.g. El Salvador). But I know of no other case of elections for each branch at the midpoint between elections for the other.