I would like to say a word or two.
I think a good way to convert your friends is simply they need quality time spent on Reach. I dunno how much was changed from Halo 3 to Reach but the point is there has been big enough changes for Halo 3 players to complain about Reach. Not everyone is quick to adapt to change. Some are real quick to adapt to the new controls and gameplay whilst others are slower to adapt.
Showing awesome footage always doesn't work. In the case of an FPS game, showing people footage of devs showing off multiplayer footage and showing pwnage doesn't always work because just because they're shown pwnage footage, doesn't mean they can just as good as shown in the footage if you know what I mean.
My advice is further enforced with a not long ago example, the Left4Dead 2 boycott. Fans of L4D didn't like the idea of a L4D 2 and hence decided to boycott the game. That happened for a long time even when Valve was showing off footage of L4D2 but the fans weren't buying it. Then Valve decided to invited the people who started the boycot to their studio and allowed them to play L4D 2 first hand and those people walked away excited and hyped about L4D 2. That is why the boycott died off as it got nearer to the release date.
You see, people are only playing a Beta and not an actual final product demo and so there's time for Bungie to make changes, balance it out, etc and eventually fans of Halo will enjoy Reach.