Gotta love 1958. And people act like losing 9 seats is a big deal.
When you lose a majority in Senate (and most chances to regain it until 2020-2022) - it's a BIG deal...
Because gaining 4 seats is so implausible. 
In present political climate it will be difficult, Besides Johnson, Kirk and (IF he runs for President AND stays in that race) Rubio's seat i don't see really top-tier targets right now. And no, i don't list Ayotte's seat or McCain's (it's Arizona after all) among them so far. If a big Democratic wave materializes - then yes.
There's no reason to think PA and NH aren't vulnerable seats. OH may be in that category too now that Strickland is running, but it's hard to tell since there's been no polls.
Democrats were rather optimistic 2 month before last November - and what happened? And while turnout in 2016 will surely be higher, Hillary is not as good turnout motivator as Obama among core Democratic constituency - minorities. She may attract some whites, who voted Republican recently, or at least - reduce their willingness to go Republican, but that's all...
Not really. Most people knew that Dems were going to lose the Senate. The only unexpected part was the size of the wave.
Please, read DKE archives. Until early September there was almost an ironclad confidence in holding the Senate. Only Montana, West Virginia and South Dakota were considered lost then. Nobody expected Iowa and Colorado to flip, most were optimistic on Alaska and Louisiana (i will not even mention North Carolina here). May be only an Arkansas too... And Democrats expected to win Kansas and something else.
Hell, read THIS forum. There were even several red avatars that were making fun of people who were suggesting that Democrats be cautious last season. I myself had predicted a very mediocre result for the GOP (they did hold Kansas, Kentucky, and Georgia, but lost NC, LA (I was way too optimistic about the Dem turnout machine down there), and I think even Alaska). When the results were coming in most of us on the IRC were like "oh sh*t oh sh*t oh sh*t" before we realized ten minutes later that yeah things weren't going to be good for the Democrats anyway.
The polls weren't conclusive either, unlike 2010 where the results were arguably a disappointment compared to the expectations and polls. Some liberals actually rejoiced that they kept the Senate and that the Democrats "only" lost sixty something seats instead of the like 80 plus that some thought would happen.
