Some of this is basic talking and listening; phone conferences will suck! Think about your typical (dysfunctional) phone conf and then slash the quality of the phone line/Skype connection. Add in the variable accents of the offshore team. Their English is 1000% better than my command of their language: if I can't understand what they say and everything has to be repeated or translated by a co-worker, meetings slow to a crawl.
You will run into cultural differences in communication as well. The off shore folks want to do your work and they have the can-do attitude of most IT workers. They may indicate understanding when that isn't really the case. They aren't going to be able to casually wander by your desk and raise questions later so you are more likely to get stuck with their assumptions. This has happened even when we have brought off-shore developers to our offices for face to face meetings.
Finally, I have found that we are often unprepared to send work to a developer in a coherent manner. That will lead to rework, increased cost and even more communication overhead as you painfully hammer out your actual requirements.