I've read more reviews on RE6 and how it's become a straight out third person shooter. This isn't really for any of the users here, but I'm sure plenty of guests come over here to see what's up.
Leon: It has a shift from a slower pace "Survival Action/Horror" (whichever you think it is) to action. Reasons are
You're in the dead center of an outbreak in a college campus in a decent sized town. You were, however aware that the President would be in trouble so it's sensible to bring your handgun (also because you're a seasoned veteran agent). For Leon's ch 1-3 it's all about surviving the outbreak Ala RE4 and a touch of RE3/CVX (Again whichever you think it is). This is also where we cross paths with Ada due to her campaign being closer to the RE2/RE3 style. Afterwards, the story shifts into a more action like feel, but not like Chris' game. This is because you're now hunting someone down to capture an possibly kill someone (Hence Leon and Helena's wardrobe change). This is also where we interact with Chris and Jake's stories due to their action like gameplay.
Chris: You're part of what's basically a military for fighting B.O.Ws. Of course you'll bring heavy duty weapons and explosives just in case they expect to handle some tough B.O.W.s or even if they have guns themselves (The Kennedy Report was sent to the B.S.A.A so Leon probably mentioned Ganados having guns and later, Chris and Sheva encounter Majini with guns, so I'm sure the B.S.A.A took extra measures). Now I've noticed that many of them say this campaign plays like Gears of war and Call of Duty. Now, I've never played Gears of War, and I find Call of Duty uninteresting, but I do know that CoD has to do with military stuff and shootout which is exactly what the B.S.A.A really is.
Jake: It's like the other three campaigns mashed into one. It has its action, stealth, and slower paced moments. But, you're a trained mercenary. You've been one for years at best to provide for your sick mother. So, as a result, you might have trained under some very good shooter and hand to hand experts so that's where the "action" kicks in. However, what should have been a simple escort ended up becoming much more complicated. All you have left are your combat skills and a seemingly rookie agent.
Ada: you're going on a solo mission, so it would make more sense to bring some decent firearms. Plus, you've been trained in the art of stealth (as far as we're aware of). You weren't expecting some idiot J'avo to foolishly hold down his trigger while flailing his arms around. Nor did you expect to get caught and experience a sinking sub or B.O.Ws powered up more than ever. Like Leon's Campaign, it has a "survival" type gameplay which explains why Leon and Ada have more intersections as do Chris and Jake.
Now as for the "Third person shooter" explanation (This goes for Leon, Chris, and Ada, does anyone really expect them to be like they were back in 1998? They're seasoned Veterans. They've learned to adapt better to seeing new B.O.Ws having been fighting them for at least 15 years. Same goes for B.O.Ws. They evolved just like the main characters have. Compared to our technology, it wasn't the way it was like 15 years ago (I still remember a few things here and there).
For an "Our-Universe" perspective, the reason Capcom decided to put four RE4s into one game with different gameplay in each is probably to test us. One day, they might post a poll on their website asking which campaign had the best gameplay. Once the votes come in, they might just put that as the focus of RE7. As for the change, genres can change. It's sometimes for better or for worse. My guess is, they did for better. I mean, what would have happened if they never changed the style? Especially for all these sequels, wouldn't it get stale? I remember watching a youtube video where someone complained about how RE3 had the dodge mechanic and not pressing a button to go up stairs and that it made the game more "actiony".
Conclusion: While I definitely don't think RE6 is perfect, I don't think it deserves all the hate it gets from some crybaby reviewers. But you won't truly understand something unless you look deep into it rather than taking in the tip only. This is why I never had any problem with the changes with the series once I finished CVX and moved onto 5. Fighting B.O.Ws for these guys is like a profession for them. You get better at it overtime. I still feel that everyone's entitled to their own opinion only if they have good reasons to back it up. While this is open for everyone to talk about, this is mostly for guests who stumble onto this thread.