OK, so Austen is awesome, right? Right. Because of that, many people who read her novels want to see in them some confirmation of their own ideas and values and moral certainties. They want her to preach to the choir – and they want to be part of that choir.
What does that have to do with the ending? Well, traditionally, the ending is where the moral of the story goes. Think about all those fables, which literally end with "and the moral of the story is blah, blah, blah…" It's not always quite so spelled out, but the general theory goes that, whoever wins the prize at the end is supposed to be a shining example to us all. Whoever gets a comeuppance is supposed to be a cautionary tale. This is easy enough with fables, where there's a clear bad guy and a clear good guy, but with a novel like this? Well, let's just say there are some disagreements about what this ending is supposed to teach us all.
On the one hand, there is the conservative argument. Now, we're not talking about conservatism as in right-wing American politicians. Here, we mean more basically the idea of conserving and preserving the status quo. Things are good now, and they were even better a little while ago, so let's all go back to that time, shall we? This line of thought reads Austen as very conservative. After all, the end of the novel, everyone is married and settled and happy ever after. No loose ends, everything packed away into neat little boxes and squared away. Men are still superior to women, women aren't agitating for rights and whatnot, and everything is hunky-dory.
Then again, there's a way to see the ending as pretty reformist and progressive. After all, Darcy and Bingley are totally upper class, what with the estate-owning and the not-having-to-work-for-a-living. The Bennets are strictly middle class, with uncles who work and daughters who won't inherit the house they live in. Still, despite this huge class barrier, the Bennet girls end up married to Darcy and Bingley. No one is scandalized by this! (Aside from Lady Catherine de Bourgh.) More than that, Darcy, Bingley, and Elizabeth all totally throw away marriage matches based on socio-economic equality (with Miss de Bourgh, Miss Darcy, and Mr. Collins, respectively) and instead hold out for spouses for whom they have feeeeeeeelings. Youth today! So disrespectful of their elders and the traditional way of doing things!