The House of Representatives is bigger and less likely to compromise than the Senate, which is where we'll see some bipartisanship (hopefully). The aforementioned liberals are angry that the Democrats compromised since as they see it, there are no moderate Republicans anyway. Actually, the House Republicans had nothing to gain by going with the stimulus the first time around. It didn't meet their criteria (lots of tax cuts, less spending), so voting in favor would have looked weak to their constituents, especially to a party that prides itself on strength. Furthermore, by going with it, it would have weakened the case of the Senate Republicans who will endeavor to develop the bill more in favor with their tastes. And if they can't work something out, Senate Minority Leader Mitch McConnell always has the filibuster option to stop a vote from happening since the Democrats are just tantalizingly short of a filibuster-proof supermajority (60, whereas the Democrats have 58, and should have 59, Al Franken, on the way).
By compromising with House Republicans even when they showed no real commitment to the bill, Democrats have built up some good ol' faith with the Grand Ol' Party. Also, if Republicans get too aggressive in their opposition, they'll run the risk of appearing as obstructing the necessary business of the country in favor of party politics--which is the partisanship we need to be afraid of. Right now, however, we're just seeing the natural, virtually harmless political maneuvering that no amount of change will ever end.