1) H1Bs depressing the prevailing wage (true if you believe in classical supply and demand)
2) overall economic utility; aka the economy does better/worse because of more H1Bs (I don't really care, but I guess someone might)
One thing which gets left out: H1Bs are more compliant employees, because their employers have them over a legal barrel. They have a much harder time leaving their employer than a citizen does, and they're a lot less likely to complain if they're abused. Turnover is hugely expensive to a software company; between that and the "costs" of having an employee with civil rights; well, not all employers are bad actors, but the temptation has to be there. I've certainly seen this at work in various places. It's pretty rotten. I mean, obviously many consider it a good trade, but it isn't exactly humane. Social costs like this are often left out of economic considerations. Who cares if your GDP goes up if we end up living in a dystopia as a result?