Just gonna lay them out here, hopefully people can answer a couple or maybe all. I'd normally test this all out myself, but I'm lacking time lately, which is why I'm also trying to be as efficient with leveling as possible.
1) Is there a game type that is best for leveling? Especially with 2XP, kinda want to make sure I can make the most of it. I've been leaning towards team-games, as there's at least a 50-50 chance of getting the win bonus.
2) Anyone actually unlock the specializations yet? Curious to see what each offers.
3) Do you gain more experience for doing Veteran/heroic on Spartan Ops?
4) I've heard there's an experience cap per day, how much is it? I'd hate to waste any 2xp on it.
Thanks for the responses--so basically Spartan Ops online with other people and in a harder game mode nets more XP? I've soloed all the content and it gave me about what I got from normal PvP matches, but took longer, so I dismissed it as a mere 'change of pace'.
1) Like truecardsfan said, Spartan Ops matchmaking gives good exp(about 4-5k usually). If you don't go with the find game/matchmaking option to pair up with 3 other randoms, it's only about as much xp as the war games playlists. That way also allows you to use double xp, while picking play chapter doesn't use the double xp. Right now in episode 2 I like chapter 3 for xp grinding as it's over in under 7 minutes unless your team afks. Feel free to experiment and find your favorite chapter though.
2) I haven't unlocked the specializations yet, but check out this page: http://m.ign.com/wikis/halo-4/Specializations It shows all the specializations and if you click on each name it gives more details about each one.
3) I haven't noticed much of a difference in the difficulty changing the exp aside from challenges that require heroic/legendary. If you're doing it for exp though the matchmaking always uses heroic so it's not an issue.
4) 150k exp as he also said.
360 games completed 100% retail: Armored Core 4, FF13, FF13-2, Force Unleashed 2, Halo 3, Halo Reach, Mass Effect 2, Mass Effect 3, The Last Remnant, SSX.