In the clip above you will see a Spartan take a flag and kill a prone opponent by stabbing him in the face with the sharp end of the flag pole. You shouldn't need anymore reason to watch than that and apparently the folks behind Halo 4 agree with that sentiment. This is an extremely short clip, and a loosely edited at that, but it makes the point early: carrying the flag in a Capture the Flag multiplayer match will be a tad bit cooler this time around.
The weak melee attack associated with carrying the flag in years past and generally feeling like a vulnerable chump while trying to secure victory are all in the past. It shouldn't be uncommon for Halo 4 players to grab the flag in order to have the chance to stab suckers with the flag. Scoring it for your team could very well become and afterthought. We'll have to see if there are other any vulnerabilities and/or benefits to being the flag carrier. An enhanced melee finishing move is definitely in the benefit column.