Paper:
In sequential tasks, planning-based agents have a number of advantages over model-free agents, including sample efficiency and interpretability. Recurrent action-conditional latent dynamics models trained from pixel-level observations have been shown to predict future observations conditioned on agent actions accurately enough for planning in some pixel-based control tasks. Typically, models of this type are trained to reconstruct sequences of ground-truth observations, given ground-truth actions. However, an action-conditional model can take input actions and states other than the ground truth, to generate predictions of unobserved counterfactual states. Because counterfactual state predictions are generated by differentiable networks, relationships among counterfactual states can be included in a training objective. We explore the possibilities of counterfactual regularization terms applicable during training of action-conditional sequence models. We evaluate their effect on pixel-level prediction accuracy and model-based agent performance, and we show that counterfactual regularization improves the performance of model-based agents in test-time environments that differ from training.

Review:
The paper presents regularization techniques for model based reinforcement learning which attempt to build counterfactual reasoning into the model. In particular, they present auxiliary loss terms which can be used in "what if" scenarios where the actual state is unknown. Given certain assumptions, they show that this added regularization can improve generalization to unseen problem settings. Specifically they propose two forms of regularization: (1) enforcing that for different actions the predicted next state should be different (action-control) and (2) enforcing that when certain parts of the low dimensional state are perturbed, over a model rollout the perturbation should only affect the perturbed parts of the state, essentially encouraging the latent space features to be independent (disentanglement). 
Overall the idea is well motivated - incorporating counterfactual reasoning into model based RL has potential to to improve generalization. Also, while the assumptions needed for the regularization to be correct are not always true, they do seem to hold in many cases. Lastly, the results do seem to indicate that generalization is slightly improved when using the proposed forms of regularization.
My criticisms are:
(1) As mentioned in the paper Action-Control assumes that at every single timestep the agent has potential to change the state. However there may be settings where the agent can always change state, but only a small component of the state. In these cases the states should be quite similar. For example a robot only moving a single object when the state consists of many objects. Also as mentioned in the paper Disentanglement will not work in stochastic environments. One concern I have is that since different environments can violate the assumptions to varying degrees, it seems like actually using the regularization and picking the correct hyperparameter to weight it will be very challenging. 
(2) The current results are only demonstrated in a single, custom environment. Additionally performance is shown on only 2 test tasks, and in all cases in Table 2 it is unclear how to interpret the reward. Does this performance constitute completing the task? What is the best possible cumulative reward in this case? The performance improvement seems small, but it is difficult to judge without knowing the details of the task.
I think the paper would be significantly improved by (1) adding experiments in more environments, especially standard model based RL environments where the performance of many existing methods is known and (2) adding comparisons to other forms of model regularization, for example using an ensemble of models. My current rating is Weak Accept.
Some other questions:
- In Table 2 does MPC amount to PlaNet?
- How sensitive are the current numbers to planning parameters (horizon, num samples)?
- Can you provide error bars for the numbers in the tables?
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After author responses and closer examination of the paper I have some additional concerns about experimental details.  Changing my score from 'Weak Accept' to 'Weak Reject'