It's not necessarily the game itself, nor the property. I mean who can argue with 60 fps and good controls and an exciting storyline? I love the game and how it feels. In my own opinion the problems with the series and why I am looking for other alternatives is simply this.
1. Activisions business decisions and pricing model coupled with them not integrating dedicated servers. Just being plain cheap and nickel & diming the consumer.
2. The community culture of this game is defiled by hackers, and cheap players in general always looking to compromise the system as a whole for their own ends at the expense of the average player. Dedicated servers could help solve this. I believe this is the main reason which sows much disinterst in it's future. By the same token these exploits are depended upon by cheap players so that they can help enable their addiction to cheap play. It's such a logistical nightmare.
3. The quality of the game from console to console is seriously slipping. The PS3 version of MW2 was a beauty as well as the 360. Somehow BLOPS was gimped on the PS3 and enhanced for the 360. Just plain lazy and is an insult to gamers like myself who own both consoles and appreciate each.
There are a few other issues I could discuss but I decided to only list the most poignant.