If you reject the Star Child or shoot him, it walks away, and Shepard is left on the Crucible watching what's left of the fleet get destroyed.
It cuts to XX, 000 years later and one of Liara's beacons is found on a planet. The cycle was lost, but Liara hopes it will be broken some day. The normal epilogue scene on the alien planet with the grandfather and the child is somewhat different - the grandfather is no longer present, replaced by a woman who has read all of Liara's archives. She tells the child they are no longer threatened and are at peace.
The cycle is broken.

A scene directly before the beam shows Shepard helping the team evacuate to the Normandy. It's by far the biggest change pre-beam.
It also varies per team member - it's actually a very nice goodbye scene that I'm looking forward to seeing again with other team members. Immediately after Shepard goes into the beam there's a scene showing the Normandy rendezvousing with Hackett's ship.

The Catalyst explains it is an AI construct created millions of years ago as the embidoment of Reaper intelligence. It was created to establish a connection between organics and synthentics, but ultimately that connection failed. The Reapers are a synthetnic representation of the first species to create the Catalyst. Apparently, they did not wish to become a Reaper...

The Crucible is nothing but a power source, but in combined with the Relays it can send energy across the galaxy - crude, but effective. The Reapers were aware of the Crucible for cycles and had assumed they had destroyed all knowledge of it in a previous cycle. It was a surprise for the Reapers to see it built. The Catalyst tells Shepard all of this because the merger of the Citadel and the Crucible has changed its programming.

In explaining the Destroy choice, the Catalyst mentions that a lot of damage will be done by using the Crucible, but no more so than what has already been lost.

In explaining the Control choice, the Catalyst specifically mentions that Shepard's memories will remain intact, though he/she will no longer be human. The Reapers will be controlled. Shepard will replace the Catalyst - who isn't looking forward to it.

In explaining the Synthesis choice, the Catalyst explains that Shepard's organic energy will be broken down and dispersed throughout the entire galaxy. The Catalyst explains that this option could not have been done in the past - it was attempted but always failed. It is not something that can be forced, only by choice. The Catalyst goes further and explains that not only will synthenic and organic life merge, but the Reaper's preserved civilisations will also be linked to everyone else.

In explaining the Refuse ending, the Catalyst explains that Shepard has no choice - all of the fleets resources was used to get to this point and the cycle will continue. Shepard refuses to believe, but the Catalyst interrupts, stating that his/her belief is not required.

If Wrex lives, then the Krogan work on rebuilding. If Wreav rules, they're arming themselves for a fight.
If Eve lives, then she plays with a young krogan with Wrex - or no-one if Wrex is dead.

I like the fourth ending, not only because it provides a bad ending that's fundamentally different from the others, not only because it allows you to tell off the starchild, but also because it gives my extreme renegade Shepard an ending that isn't all sappy and "he died for us".  Either I'm surviving, or I'm taking you assholes down with me.  Blaze of glory, bitches.

Okay, now that I've played through and destroyed the Reapers I am by and large satisfied. I'd like to have seen a bit more regarding details of what my squadmates did post war (like a text crawl), but if this had been the original ending to the game I would have been fine with it. I also would have liked to have seen Shepard have a reunion with at the very least his love interest if he survies (that breath scene is still in the ending).

I also have to ask - why did they even leave in the scene with Buzz Aldrin and the Child? It's not at all relevant witht he new endings? I also thought it was a nice touch/good PR move by BioWare to change the message at the end that was previously designed to sell downloadable content.