I liked the base combat in Halo 4, the DMR felt great to use in the beginning... But over time, it became clear that the changes 343 had made were too extreme. They fundamentally changed the way Halo is played, and not for the better.
Matches quickly swing in favor of the stronger team, as they are awarded an endless stream of power weapons that become easier and easier to obtain. By the end of the match, one team is running around with snipers and rockets while the other sits helplessly, getting pounded into oblivion with no chance to strike back.
Instead of emphasizing teamwork and map control, it's merely a race to get a handful of kills and call in ordnance as quickly as possible, regardless of the game type. In that way, it is no different than Call of Duty. The objectives are merely a distraction in what is ultimately a game of attrition.
I sincerely hope that the comment about arena mode is proof that 343 recognizes this to at least some extent, and is taking steps to correct it. Removing armor abilities is definitely a huge step in the right direction as well.
Depending on how powerful the ground pound is (if that's actually what it is), it makes me a little worried about melee combat, being one of the original three pillars of Halo gameplay. Maybe I'm over-thinking this and it's not going to be a big deal, but 343's track record thus far does not exactly inspire confidence...
The slide I don't really mind. It doesn't break map traversal like the jet pack, and merely serves as a quick way to get into a crouch and maintain your forward momentum for a little while.
I ended up rambling a bit, but those are my slightly-sober, mostly-delirious 5am thoughts...