One problem with Wii Speak is that only one person can talk at a time or at least what I have experienced with Animal Crossing online. If both people are trying to talk at the same time it is just a mumbled mess with no one understanding each other. I am not sure how it works with 360 or ps3.
Another problem is that some people's mic pick up on their tv surround sound or just tv speakers. You hear a lot of echoing which the Wii Speak Channel tries to fix but it is still present for some people (looking at you john_lucas!).
If they include Wii Speak in any game there always should at least be an option to turn it off due to people not wanting to hearing little whining kids or someone's messed up mic. Another option should allow you to choose who you want to mute and who to hear which isn't available in Animal Crossing.







