As usual, this comes with good, bad, and ugly sides.
THE GOOD, for Democrats anyway, is that the timing of the announcement couldn't be better. Nevada is a razor-edge purple state such that Reid would have lost his last election (in 2010) if not for his GOTV machine. 2016 is a much more favorable environment for Democrats, as long as Hillary Clinton runs and the usual groups (OFA, DFA, MoveOn, etc.) are firing on all cylinders. They also have a popular candidate in recently retired Nevada Attorney General Catherine Cortez Masto, whom Reid endorsed today.
THE BAD for Democrats is that this gives the GOP an opening to take this seat. Right now the GOP have a great candidate in the current Governor of Nevada, Brian Sandoval. He's enormously well-liked, and if he runs, even with all the advantages Cortez Masto has, it's going to be a toss-up. If he doesn't run, Cortez Masto is an easy favorite. So the GOP is going to work very hard to recruit him.
THE UGLY is that Reid has already endorsed his pick for the next Senate Majority Leader as of 2017, Chuck "I Love War and Gun Control" Schumer. Even without this endorsement, Schumer has seniority and is the most likely pick. Elizabeth Warren, the progressive favorite, has less than four years of seniority, so you can forget about her having a shot as Leader for a while. Those groans you hear are partisan Democrats being forced to bite the bullet and swallow yet another move by their party to the neoconservative center-right.