The Lib Dems could have stopped them, after they got the tories stuck in a fixed term parliament, the tories needed them to get anything done. A strong leader could have stood up to the tories a lot more, they got annihilated at the next election because they didn't. They didn't get a single one of their policies that the general public actually cared about through intact. aka the general public really really really don't care about constitutional reforms and how we vote politicans into power in this country. An even the referendum on changing our electoral system was watered down by the tories to the point where there was no point in changing it.
An it happens in similar coalitions all across the world, minorities getting at least one or two of policies fully implemented. Ultimately Nick Clegg will go down as one of the worst politicans this century, David Cameron and Theresa May won't be far behind them.
Gordon Brown will at least be remember for taking leadership during the financial collapse in 2008 an leading the world to solutions to stave it off. Through at the bottom it will probably have something along the lines of the bloody idiot called a election either three years to late or two years to early, both would have led to election 2012, where the economy the on the up, as the tories policies didn't completely kill off growth.
How Blair will be thought off will ultimately depends on how Middle East turns out in the long term. I suspect in the fullness of time he may actually be considered someone who change the middle east for the better. People in the future won't care about how he lied to get it done or how many people died because of his actions, after all we don't care about how churchill sacrifice Coventry to hide the fact we broke the german codes or the devastating bombings of Dresden, we just care that he lead us to victories over the Nazies. An ultimately I believe Iraq change the middle east in ways that we won't see the full extent off for decades in the future.