I don't agree that Halo Reach and CoD Black Ops need to be radically different from their predecessors.
There seems to be this assumption that shaking up the formula would be a good thing in gaming right now, but I couldn't agree less. I tend to equate these things to movies so I'll take some good examples.
The Matrix shook things up radically for the sequels, allowing the philosophical undertone to become the focus of the movies instead of the action sequences which had made them famous. They brought in a lot more characters to meet and greet, the dialogue took up most of the movie and showed that actually it was never the best written story ever. The seuqles were critically panned and generally publically panned as well.
The Terminator films kept things pretty much within the same context, robot soldier comes back from the future to kill the human resistance before it begins, good soldier sent back to prevent it from happening. That is the story, both times, of the first two movies. You know, the ones that did well and everyone praised for the second one being better than the first.
Keeping things similar is the best way to keep an audience. If Halo and CoD had changed anything dramatically they would be getting pilloried for having changed too much from their original material.
Game developers are damned if they do and damned if they don't, but at least if they stick with what's familiar to the majority of their customer base then they will still sell games, and the only people who will care are people who come and whine about it on internet forums.