This was recently brought up in the comments, so I figured it worth an entry.
Okay, so right from the get go, I don’t think you should ever state that a work is bad before it comes out. Nor do I think you should state that a work is bad without first having read/watched it (I think I’ll even extend that to “having seen promos/trailer for the work”).
However, I think “that looks bad” is perfectly reasonable if you, well, think it looks bad. Likewise, I have no problem (heck, I do it myself) with predicting quality of future works based on information you’ve previously acquired from past works, so long as you are only PREDICTING quality rather than making declarations of the quality of the work.
If I pick up a Jason Aaron comic book, before I open it up, I figure that the odds are that it will be well-written, based on his past work leading up to this new issue. On the flip side, if I pick up a comic by (name withheld*), I know the odds are that it will be poorly-written, based on his/her past work leading up to this new issue. So if I wish to say something like, “I am not looking forward to Writer V taking over Book W” or its counter, “I am looking forward to Writer X taking over Book Y,” then I think that is completely acceptable.
A slightly more problematic point is the question of citing the critical consensus. I think it is acceptable, so long as you are pretty clear that that is what you are doing – citing the critical consensus, not stating your stance on the matter. So if a comic is getting totally panned by the critics, I think saying “Apparently, Comic Book Z is not very good” is acceptable, but yeah, probably better to be clearer, just to avoid any possible confusion.
*The name wasn’t really withheld, that was a joke – I didn’t even think of a particular name.