The left is far too willing to make abortion a black and white issue (the right is too, imo), and refuses to consider any kind of nuanced position on it. Same thing with immigration, you're either for complete amnesty, or you're Donald Trump according to many leftists. Although I disagree with most leftists on marijuana legalization, their arguments and viewpoints don't frustrate me as much as their viewpoints on the other two issues.
Also the minority of Democrats who support expanded free trade and are anti-union. But I wouldn't necessarily consider those folks as being on my "side".
I agree with you that the issues of abortion and immigration are often turned into a one-position-fits-all stance for both sides.
I would love to see Dems take a more nuanced and careful position on trade policy and other economic issues. Republicans too for that matter. It seems all to often that members of both major parties seem to be all too accepting of what the party's main stance is, and do not think critically about the policies of said parties.