One of the biggest (and frankly, only) faults I've found with Barack Obama's Presidency has been a relative inability to deal with Congress. While the issue is understandable (considering Republicans in Congress have had it out for him since day one), we can't ignore the lack of progress that this issue has caused. Let's also not forget that even with large majorities in a Democratic Congress, much more progress was feasible than what was delivered.
Considering that at least the House is almost guaranteed to stay in Republican hands until redistricting, I think an ability to successfully get things through Congress is an essential quality in the next Democratic Presidential nominee and their team.
At the end of the day, we elect a President, not a king; why should I believe that electing Bernie Sanders will result in the groundswell of progress his Presidency promises? Why should I believe that Sanders initiatives will pass Congress and become law? Saying he's better than Hillary isn't an acceptable answer for me; I may very well vote for him for that reason, but I'm asking a question on his merits right now, not hers.
For the record, I was an avid Hillary supporter until recently, when I came to like Bernie enough to become undecided.