Namely, whether they will turn out to be a gold mine for content scammers in the short run, poisoning the well for actual authors of any sort in the long run. Jim Macdonald covers it all, with relevant links, over at Making Light, and rather than duplicate efforts, I’ll just commend that link to you and tell you to follow it.
I will specially highlight one thing Jim calls out there, which is that some assbite has taken an entire novel by author S.K.S. Perry and put it up on Amazon’s Kindle store without his permission, and Perry’s currently wrangling with the store to have the pirated version taken down. He’s in the unfortunate position of being a pioneer in this particular case. He’s unlikely to be the last.
This is where a DMCA take down notice does come in handy; when presented with one, Amazon (and most other legitimate commerce sites) is legally obliged to deal with it, and stuff like this is what a DMCA notice was made for. If you’re an author and you’ve not seen a DMCA notice before and/or need an idea of how to put one together, SFWA just happens to have a sample DMCA notice generator for your informational purposes.
This is where I note that DMCA notices have to come from an author or his/her duly-appointed representatives (agents and publishers), so you shouldn’t be sending them for a pal or as a pre-emptive nerd strike in defense of your favorite authors. You can get in a fair amount of legal trouble for that. But if you are the author (or his/her duly-appointed representatives), then it’s a useful tool.