I don't know if this is a bug or something Bungie intended, but if you're playing through the campaign with scoring turned on you'll miss out on the credits and the brief cinematic that goes with them. I don't want to get into end game spoilers so I'll just say I found it odd when I finished the game without ever seeing the credits and when I checked online I found this from bungie.net:
 
 I didn't get to see the end Credits!
Likely you have campaign scoring turned on. You should complete the game with scoring turned off to see the game's end credits. 
 
 http://www.bungie.net/News/content.aspx?type=topnews&link=Reach_FAQ#_777
 
If, like me, you missed out on that part you can find the bit you're missing on YouTube.

I noticed that too, thanks for this.

Well, fuck. I've been playing it on and off with scoring. Is it just really the last level you play that matters? Like if I turn it off before beating it, I'll get the credits sequence? 
 
Either way, my campaign progress is going reaaaaaaal slow. Multi's too addictive!

Wow, that's strange. I can't honestly see why you shouldn't be allowed to see the end game credits if you're using scoring. Maybe it's a way of making the game that little bit longer? I'm not really sure, but it does seem like an odd decision on Bungie's behalf.

Thanks for the heads up, been playing with it on and only 2/3 levels away

Thanks for the heads up! 
 
After finishing the campaign on legendary with a buddy of mine a few nights ago we were both like "wtf?" when it just ended with no credits. 
 
Do you still earn cR points with scoring turned off though?

Wow, i never knew this, completed the game today and thought it ended quite sudden, no credits and what not.