OK, so I guess I'm quite a forgiving person when it comes to this series and I really enjoyed Halo 4 (yes, I am a Halo fan), so I'm always going to be a glass full kind of guy when it comes to Halo games as I love the series so much.
The pacing of 5 was spot on, IMO. When I started off, I was a little disengaged by the whole thing and the first few missions felt like a bit of a plod but it started picking up - the back and forth between Chief's and Locke's teams worked VERY well and I think added to the storytelling.
Gameplay wise, I mean, with the newer style controls and sprint and ADS there is more of a temptation to just run and gun through levels, but you'll quickly find that tactic won't get you anywhere. Even on Normal. This is still Halo.
The biggest disappointment for me was your AI team-mates. They are dumb as hell. Sure you can control them to go here and there, attack certain things, pick up certain weapons, but if you go down and you ask them to revive you, they run up to you, regardless of what enemies are around and go straight to get you up - no teamwork to take the enemies out etc. It made them feel like not a team of Spartans at all.
That was my only issue with the campaign though, as frustrating as it was. I loved the story, the pace, the gameplay. I'm not a huge fan of the Promethean race personally but hey ho. I thought the actual ending was a little lame, could've been expanded a lot more, but as I said, it's a setup for Halo 6.