The 538 forecast is holding steady at an average 39 seat Democratic gain; other forecasts, while typically lower on seat gains, are also holding steady on a Democratic takeover of the House.
So, what could go wrong? I already covered this a bit in a previous post which dwelt on the inherent uncertainties of probabilistic forecasts and the known unknown of the extent of Republican counter-mobilization in red areas.
But here's another way of looking at it: considering the floor for Democratic gains. That's basically what this latest analysis from Sabato's Crystal Ball does--it enumerates a number of places where the Democrats seem almost certain to pick up a certain number of seats. That number comes to a (net) of 17 seats. That's not enough to take the House--if all the other many, many possibilities the Democrats have don't pan out, a worst case scenario and not at all your "expected value". But it could happen (though 538 puts the chance Democrats gain less than 19 seats at 10 percent). And if it did happen, the distribution of seat gains for the Democrats would probably follow the floor outline in the Crystal Ball piece.