No, Halo hasn't changed much over the years. Well, it was aimed at a specific demographic to begin with, so to change it into something it isn't would be a stupid thing to do at this point. Also... you might find it actually HAS changed beyond a little bit. I shall now go to lengths to explain how, and why Halo is so great anyway.
Graphics. Don't get me STARTED on how nice and vibrant the multiplayer levels look. Now, have you haters SEEN the campaign?! Its levels give off a totally unique atmospheric effect that really makes you feel like you're fighting a losing war to defend the planet. The Reach campaign really sells the emotional value of the covenant destroying a really beautiful world. Maybe you need to pay more attention and just let the story take you in.
Yes, CoD gameplay changed dramatically. That's because Activision realised they needed to get it up to speed with the times. The Halo franchise, however, has never really NEEDED to change - its gameplay has always been different, unique to that series, and most importantly it works for the demographic that it's aimed at. And yet somehow, despite it's lack of change, the franchise STILL rivals the CoD series. Anyway, despite there not being any such NEED for change, there's definitely been a LOT of it since the first game. You all just don't want to admit it.
XBox Live, for one, from Halo 2 onwards. Halo's 3 and Reach have Forge, a function where you can create your own maps for multiplayer madness on XBox Live. I've hosted a lot of parties with custom maps and gametypes, and they were insanely fun. Reach then took the Forge and improved SO many of its features with exact positioning, even more basic objects, and not to mention Forge World, a map that is RIDICULOUSLY huge made almost especially FOR Forge. Then ODST introduced Firefight, a mode which is almost a semi-campaign in that you have to hold out with friends against wave after wave of Covenant forces and score as many points as you can. Reach took that awesome function and made it a matchmaking success. Also, a lot of technical kinks that annoyed players in the previous Halo games have been ironed out in Reach. Any frustrations that keep people from scoring kills now are just excuses to make up for them not being able to play the game right. It takes practice, like any good game, to be good at it.
Halo has an AMAZING storyline in an IMMENSELY expansive world, and Reach is no exception. The series has been adapted into many successful novels, comics, webseries (like Red vs Blue). I've even seen it made into board games (and it's even got it's own version of Risk). You don't often see this level of merchandise and spin-offs from a VIDEO GAME. I'm just waiting for the day when Hollywood decides it's a goldmine for money and makes some films.
As for Customisation... Well I already ranted about Forge, so I won't repeat myself there. Character-wise, In the original Halo game you had... what? Just a block colour that you could apply to your Spartan, which looked exactly like Master Chief no matter what you tried. That's boring. Reach, however, really lets you get your teeth into creating a new character, with individual armour pieces, primary and secondary armour colours, a 4-digit service tag, character voices, male or female, Elite or Spartan... the list goes on. You can also customise just about every little detail of game variants too to play with your custom Forge-made maps, or even the preset ones. Firefight customisation has also been improved massively since ODST.
So really, think about it. The game hasn't moved on at all? Stop and think: A) Is that so true? B) Did it ever need to fully change it's gameplay anyway?
5/5 by the way.