I like how people ignore the actual hard data to whine about "unskewed polls" or whatever.
These are facts. Nobody disputes this. Does this mean it's guaranteed to happen again in 2014? No. Does it mean it's more likely for polls to underestimate Democrats than to underestimate Republicans? Certainly.
I think the bottom line is the "likely voter" screens. While LVs are much more reliable than RVs close to an election, an election isn't just likely voters. It's likely voters plus some "unlikely voters". These "unlikely voters" tend to skew Democratic, thus Democrats outperform the polls. It also depends on the pollster. Most of them don't even bother to disclose their likely voter screens so we have no idea how strict they are. Some of them have absolutely ridiculous ones like "MUST have voted in 2010" or "MUST have voted in the last 10 elections" which are obviously going to give heavily Republican results.
