As a long time freelancer with a few purchased gigs as well:
Quality of the workers or the turned in work is directly linked to the budget of a project. I've had many conversations on both sides of the table, where there was a miss-match and an eventual agreement to either bump up the budget or to lower the expectation. There's still risk in eventually receiving lame work, but
I don't think that is the status quo.
There is a larger than obvious amount of bot or automatic submitters tainting and lowering the overall quality of the pool. It is very easy to sort them out, but it is noise nevertheless. Quality filters and machine taught spam detection would deal a great boost to overall usability of all of these sites.
Now, communication and missed deadlines will deviate wildly because most of the developers are not able to sustain a living doing just elance, odesk or whichever. Honesty and setting expectations at the beginning usually solves this problem. Now, there has been a steady build up of downward pressure from contract owners to expect full availability or a similar version there of. Most of them don't have the budget to back it up. Shit happens.
I found and worked with talented folks through 3 of the 4 mentioned sites and I would have to say that it is not an issue. The issue is actually finding them in short time, and that's where buck stops in my point of view.
I think there is a large opportunity to impress the market by shaving off the amount of time to find quality devs and firms. I think other sites have began doing something along those lines by curating the offers and doing away with the meat-market environment.
Overall, I think the budgets are low, developers pick up multiple gigs, some projects get delayed or rushed and compromise is the name of the game.