The Daily Post's editor, Dave Price, does some great work analyzing local elections–candidates and the process.  His editorial yesterday noted that the move to an all mail-in election gives an advantage to incumbents.  He writes

If cities and school districts agree to participate in the experiment, all ballots would be mailed out 29 days before the election.  Each city would have one polling place for people who want to cast their ballots the traditional way at the polls.  But the idea is to get those ballots out early and get people to vote early.

That provides an incredible advantage to incumbents.  In the early days of a campaign, people who vote tend to favor the incumbents…In the final weeks before election day, challengers hit their stride and start raising issues, challenging the records of incumbents.

And I would add there is more news and editorial coverage of races, more chance for public debates and more door knocking time.  Price provides some evidence from last fall's Menlo Park Council race.  He goes on to note that for various legitimate reasons unique to mail-in ballots

UC Davis research shows that 3% of the vote-by-mail ballot in the state's 2012 general election were not counted.  Three percentage points is often enough to throw an election one way or another.

Here in B'game we don't get decisions by 3%!  More like 9, 9 or 77 votes.