Lots! As noted here, the House forecasts are uniformly very good for the Democrats, with average seat gains estimated in the 31-39 range and probability of House takeover estimates in between 70 and 84 percent. At this point, 538 is on the high end of these ranges.
But there are some problems to keep in mind. The first is more accurately thought of as not a problem, but intrinsic to the enterprise. If, say, there's a forecasted 4 in 5 chance the Dems will take the House, this directly implies there's a 1 in 5 chance they won't. So things could go "wrong" in that sense.
Second, there could be a systematic bias in the inputs to these forecasts--something that's not being captured or captured incorrectly. The most likely candidate for that "something" is a mobilization surge in red areas that is currently being underestimated. We've already seen some deterioration in Democrats prospects in red state Senate races; it is possible that in the redder congresssional districts Democrats are contesting that the same dynamic may hold down their gains in such districts.
This is really the point of Nate Cohn's article in the Times today. Note that Cohn still appears to think it's likely the Democrats will take enough seats to capture the House, but he does see the possibility--and some signs--that this mobilization dynamic will hold down the magnitude of Democratic gains. I would not say that polling analysts universally see the signs that Cohn does, or interpret them in quite the same way, but I do believe his caution should be taken seriously.