I need to preface this by saying that what I'm about to say is my personal opinion as a player, not a moderator, and doesn't necessarily represent the views of the staff or forum as a whole. Stuff I like about the beta, based on what I've played of it so far:
The weapon balance seems decent enough, and in particular, it's really nice to switch to an Assault Rifle and actually expect to do something with it. Bungie had an odd obsession with making automatic weapons near-useless compared to precision guns even at their supposed optimum ranges, and so it's great to see 343i continuing what they started in Halo 4 by trying to make all elements of the sandbox viable options.
The new movement options feel really good: clambering felt completely natural from the very start, the thruster pack actually feels useful without being overpowered, and while I haven't used the slide function much owing to my weird controller setup, on those occasions where I have tried it out it's been both very useful and kind of cool.
Sprint seems balanced pretty well, and with the addition of the thruster pack the shield recharge delay doesn't completely ruin player mobility after taking damage.
The power weapon and enemy position callouts are helpful without being overbearing, and the inclusion of named, visible position callouts from the start of the game is going to be very nice from a competitive perspective.
Stuff I don't like, based on what I've seen:
Killcams. These weren't such a big deal in Halo 4 as they were easy to skip, but for some reason unless I'm mashing my X button like crazy I can't pre-emptively get rid of them. An option to toggle them off altogether would be very nice: on a related note, I'd far rather just stare at my own corpse for five seconds after dying than have it automatically follow a teammate. It's really disorienting and I'd far rather know who killed me and have a few seconds knowing where they're going/what they're doing than killcams and teamfollow, neither of which are any use whatsoever.
ADS Smart Scope. I complained about exactly the same principle when the first screenshots of Halo 4 came out, because it makes the screen incredibly busy and distracting - and that was when there were just two small HUD markers around the reticule. Now the whole screen is taken up with crap and it makes it far harder to acquire and track targets than previous scopes: there's also now literally no point to scoping in the pistol as the 'aim assist' holographics are literally so obstructive that it becomes just about impossible to actually see the target. I don't hate this because it isn't 'Halo', but because it's a horrible, cluttered method of aiming and targeting in every game it's in, and I'm desperately hoping there's some form of more classic aim introduced for those players like me who don't like half their screen being taken up with distracting nonsense. There's also some evidence - though I can't confirm this for certain - that smart scope and hipfire actually DO have drastically different bullet spread profiles based on a few tests I conducted when I fund a quite spot on Truth for a few minutes, with the AR in particular becoming incredibly precise in its bullet spread, so I'm hoping that's fixed before release. As is, there's a real need to use ADS in order to be competitive, and combined with the horrible animation and screen-filling crap that comes with doing so, that's a big issue.
On the same theme, I really don't like how big and screen-filling the weapons have gotten. Guns in Halo have always been pretty large on the screen, but the DMR in particular is now enormous and distracting as hell. I'd love to see weapon models toned down a little bit pre-release, although admittedly the transition to full 1080p might resolve this naturally.
There's a LOT of verticality in the maps I've seen so far, and yet inexplicably the reticle has moved back to the centre of the screen rather than staying in the bottom third where it's been for the previous ten years. There was a very good reason why it was there, and that's because it's much more useful to be able to see what's above you than it is to see what's directly under your feet. Put it back where it was. Seriously.
The FOV seems a bit... off. This might be tied into the fact that the reticle has moved and the gun models are higher and oriented differently from before, but if it could be moved back to the H2-H4 standard rather than that of CE, which it roughly seems to be analogous to now, it'd be nice. This is doubly important given the complete mess ADS makes of the screen - it's bad enough that half the screen is obscured by crap without targets being half their size in other games.
From a texture and lighting perspective, the game is complete crap. This isn't a huge issue yet as it's still very early in the development cycle, but I really hope someone takes a good look at the lighting and colour palette in particular before launch, as right now it's often quite difficult to see opposing Spartans depending on the lighting - Truth in particular gives blue team an enormous camouflage advantage due to its lighting, so either revisiting lighting or making armour a bit less saturated and plasticky would be great - Halo 4's light coloured teams were perfect for this kind of moody background, and so I'm hoping we'll see a return to that kind of colouration.