Obama ignored most of the issues they really cared about (Ending wars, legalizing marijuana, closing Gitmo) and his effort on dealing with education costs in any meaningful way was tepid at best.
Obama campaigned on winding down Iraq, which happened, and ramping up Afghanistan, which happened. He did exactly what he campaigned to do. Is it unpopular now, yeah, but we're so entrenched in Afghanistan there's no good way to pull out where things won't immediately go to hell.
On his second day in office, Obama signed an executive order to close Gitmo. Congress outmaneuvered him, bolstered by DINOs and Blue Dogs, and made it illegal to fund any relocation or closure efforts. From then on, prisoners and detainees have been stuck in limbo. Most of them, even if found innocent, will be unable to unwilling to go back to their home countries. Gitmo is a clusterfuck, but not of Obama's doing.
Marijuana legalization? Yeah Obama's a big ol' hypocrite there.
Education costs? The federal guaranteed backing of student loans is the exact thing that's responsible for the huge cost of going to college. Once banks know that they won't be on the hook for nonpayments, they started handing out loans like candy. Colleges, now flush with applicants, simply raised their rates and funded massive white elephant projects. In my city, each of the major universities are undergoing major facelifts and additions worth hundreds of millions of dollars. The notion that every student needs to go to college is stupid. Most kids would be better served to go to trade schools.
Internet freedoms, which you didn't mention but is a huge issue for younger voters, is another area where he's completely failed. He's tilting at the MAFIAA's windmills.
All that being said, I'd rather have another 4 years of Obama than allow Romney anywhere near the White House. Obama's had his failures and disappointments, but he's had areas where he's done well or at least well enough to deserve another four years. But still, if things continue polling how they are in my state, and he's assured to capture my state's electors, I'll be voting for
Jill Stein anyway.