I think there was a big problem with the fact that a lot of his voters really bought into his rhetoric at first, and when they ran out of justifications and convoluted reasoning to explain why it was fine that they weren't getting whatever they expected with "hope and change", they became disillusioned with the Democratic party. They were unwilling to vote for anyone other than perhaps Bernie Sanders at this point, or perhaps they might try an outsider Republican who all the respectable people dislike. I don't blame them for getting disillusioned, I am however tempted to blame them for unrealistic expectations, and voting for Trump, if that is what they did.
My personal stance is that I believe Obama's presidency was the most overhyped thing I've ever seen in my life, even more than The Phantom Menace. The amount of "just you see', and "just you wait", or "you gotta believe", or outright anger I got from people at expressing my skepticism was exhausting. The international community did not help with this, either.
He seemed ok to me, but I didn't see why everyone was treating him like this messianic figure.
I did give him a shot for 8 years and only challenged him on 1-3 things. I think when you campaign on vague slogans that people take to mean whatever they want it to mean, and then you run a relatively restrained and conservative government that doesn't change that much, it's not surprising that there was backlash, which might have manifested among some people as a vote for Donald Trump.
I think he had skill as a politician, but other than that I was skeptical of him to begin with and he let me down on a number of occasions even so. For instance, I thought that at the least we would have more unity, as he talked about in his speeches, but the last two years of his term were extremely divisive, and you can't pin that all on Republicans (I will not speak of this more because as I said above, it's like kicking a dog when it's down).
Sanders would not have been able to do everything either, but I feel like he would have been much more unrelenting in his efforts (I sense Sanders is an NTP and they can be unrelenting if they actually give a damn about something) without making these speeches that made situations worse in an attempt to be everything to everybody. And again, I like that his motto highlighted that it wasn't just about him, it was about all of us. It wasn't centered on this singular hyper-intelligent hyper-competent transformative figure to whom we owed our allegiance.