Maybe they have particularly shitty, untenable ideas?
But they're not! Democrats have totally passed a whole slew of their own legislation against Republican opposition. They also sometimes pass Republican ideas (like the ACA).
For example, when Democrats had full control of government from 2008 to 2010, they passed a whole slew of legislation related to women's rights (e.g. the Lilly Lidbetter, with all but 3 Republicans opposed), economic reform (e.g. the economic stimulus bill, 100% of Republicans opposed), and gay rights (e.g. the hate crimes prevention act, opposed by most but not all Republicans). They also officially ended the Bush-era torture tactics, negotiated some new arms reduction treaties, and banned gifts from lobbyists to all white house staff. I think these were all good things, and an ethical Republican administration could have adopted at least SOME of those Democratic ideas, if they weren't so worried about opposing the secret Kenyan Muslim traitor in the white house at all costs.
But they ARE worried about it, so they can't adopt any of those good ideas. It has nothing to do with whether or not the Democrats have good ideas to offer, and everything to do with feeding red meat to their fringe base constituents.