1) The magnitude. The US government will spend approximately 922B of our money. This is approximately $300,000 per job created (assuming that it creates 3M completely new jobs). This is approximately $25,000 per citizen (assuming 360mm citizens), and about $75,000 per household. I'll stimulate the economy myself if you would just give me the money, while also making it easier for households to deleverage. It would help retailers, real estate prices, car companies, and banks, allowing consumers to put their money where THEY need it to be fixed. GM will survive if people buy their cars with the 25k, and not based on how good they are kissing a Senator's ass.
2) The composition. The stimulus package isn't being used to pave roads and educate our children. We already pay taxes for that, and we spend nearly the most money in the entire world on education. Instead we're spending money on broadband internet in rural areas (7.5B), giving money to the Census bureau for cost overruns (1B- and zero jobs created), and there are dozens of other little projects like this.
3) We're putting further pressure on US debtholders, and putting further downward pressure on the US dollar, eroding our purchasing power.
4) While not a big concern now, actions like this put further pressures on the monetary base, and seed the soil for inflation and inflationary expectations. Inflation acts as a hidden tax eroding our purchasing power.
5) The long-term costs of such a bill, and the amount of capital and investment that the federal government is using, is obvious to both supporters and opponents. Supporters say that the cost of inefficiency is worth the short-run improvement in unemployment. Opponents do not agree, or find this tradeoff is not acceptable when considering the above shortcomings.
Liberals in Congress would do well to engage these objections if they wish to actually garner bi-partisan support, but I suspect that is not the goal. Instead, the goal is to push through a bill that you like, and try to work the PR so you look good whether the economy recovers or not. As an example, I believe liberals used to call this "scare-mongering:"
“If we do not move swiftly,” the president said, “an economy that is in crisis will be faced with catastrophe.” He added, “Millions more Americans will lose their jobs. Homes will be lost. Families will go without health care. Our crippling dependence on foreign oil will continue. That is the price of inaction.”