Let me start off by saying this: I like Halo. Like, a lot. One could even say that I love Halo, if they were so inclined. 
 
Now, Halo has been catching a lot of flack since Halo 2 for being a playground for kids between the ages of eight and fourteen. Collectively, people's opinions about these kids are negative. They are loud and annoying and obnoxious, and that is pretty much a fact. The easiest way to avoid them used to be avoiding Halo, but as time has worn on and Halo has become less popular all around, I think these kids have mostly left for the next big lowest common denominator shooter, Call of Duty. 
Now, I think it's status as being "for whiny kids" etc. is a bit undeserved. I was actually playing Reach today and didn't encounter a single obnoxious person. As a matter of fact, I didn't encounter a lot of people in general. Considering Halo 2 had people hooked right up until the original Xbox's Live service was disconnected, that's a bit alarming. Early this afternoon, some of the gametypes only had a couple hundred people playing them. So what exactly is happening to Halo? 
  
In my opinion, three interconnected concepts are weighing Halo down. They all have largely to do with the fact that Halo adheres to some outdated, and frankly questionable design choices. Before I list them, I just want to let you all know that I don't want Call of Duty in my Halo, but I think that, by and large, it would convert a non-inconsequential amount of people back from Black Ops. Anyway, in no particualar order, here they are:

PROBLEM NUMBER ONE - CONTROLS, STAGNATION, ETC.
One of the problems with Halo is the fact that it's not "going anywhere." People these days all want what the current most popular FPS has, namely iron sights and sprinting that isn't completely fucked. The bigger issue here is that Halo: Reach, despite looking and playing better than it's predecessors, is in fact, almost completely identical to the earlier Halo games. 
 
PROBLEM NUMBER TWO - OLD HABITS DIE HARD 
Ties in to number one, and personally, this is the one I have the most trouble with- the matchmaking is a harrowing experience. What seemed slick and streamlined back on the original Xbox with Halo 2 is now nightmarish to behold. Searching for games takes way longer than necessary, and for no particular reason. And for whatever reason, Bungie won't let people join matches in progress and almost always penalizes people for quitting early. It's so easy to bring this system up to date! This seems like a huge missed opportunity to me.
 
PROBLEM NUMBER 3 - ALIENS! AND OTHER CLICHES 
The other real issue I can think of is the fact that Halo has mostly run its course as the "cool thing," which is totally understandable. At a certain point, the aliens and humans simply have to stop fighting, because let's face it: constant attacks on earth get boring after a while. It's for this same reason that eventually Call of Duty will get boring, variety being the spice of life and all. 
 
My point is thus: Halo is a great game and a landmark franchise, but as far as I can tell, the series is having trouble letting go of some of the series' dated concepts.