The first is that apps are laggy or jittery during scrolling or animations. This is partially due to poor coding from developers, but the main reason for its existence was the lack of hardware acceleration on Android devices. This changed with 3.0 on tablets, and now 4.0 on everything. Developers still have to opt-in with a one-line XML file change, due to compatibility issues. So in essence this has been solved across the board.
The second is what a lot of users are now referring to (and was largely masked by the presence of the former problem). And it's that there's a much larger drag distance required to "initiate" user interaction on Android than iOS. When I pick up a friend's iPhone, it seems like if a hair on my finger even grazes the screen the UI immediately (and appropriately) reacts. On every Android device that I've ever used, it takes approximately 0.5 to 1.0 centimeters before the UI will begin to react and scroll. This is what people mean when they say that it feels like you're "pulling" the UI on Android, while iOS has 1:1 natural interaction.
Is it annoying? Yes. But after moderate use I stop noticing it, until I happen to touch an iOS device again. Now that Ice Cream Sandwich has taken care of every other fault with the platform I had, this is really the last item on my list of complaints, having used Android devices for years on a near-daily basis. I don't know if it's a problem with the hardware of the screen sensors themselves or with the levels of abstraction to Java-land, but this should be their top priority for what I can only assume they'll call Jelly Bean.