Took a break from studying Roman history to play a couple of quick matches, so here are my initial impression based on on my short time with the game:
First, I think I should say that I have never been a great Halo player, nor have I ever invested a significant amount of time in the multiplayer side of the game (~200+ Halo 3 matches with maybe ~200 for Halo 2). Second, its been a while since I have played Halo 3 (or Halo 2 for that matter) so my memory might have been a bit foggy going into the game, plus I am recently having another playthrough of Mass Effect 2 which is no doubt further distorting any impressions I have. None the less, to say I was merely excited about playing the game would be an understatement. Now that those issues are addressed, I can get on with my impressions.
The most immediate change that struck me when my first match began was how different the game "felt". In particular, I found the movement speed to be more measured, but not laborious, just it felt like my character was moving with a bit of weight. This feeling translated over to the general gunplay mechanics of the game. If the actual movement speed of the game has been reduced in comparison to Halo's of past is beyond me, but I feel that the game places a far greater emphasis on aiming and shot pacing than the previous entries in the series. This feeling is partially influenced by the onscreen reticule which now expands and contracts to indicate the accuracy penalty you will face the next time you squeeze off a round (or two). This relates back to the more measured feeling the game has, as with past Halo games I found my self firing in rapid succession feathering the trigger on intuition (pow..pow..pow) while with the Reach Beta I noticed my self watching the reticule more leading to a slower shot burst (pow....pow....pow).
Anyway, I need to get back to studying, but I will probably post more impression on Armor Abilities later on if I get the time.