Resident Evil 6 is an action game, you'll have to accept that. Also like RE5, it's designed mostly as a co-op game, so I highly suggest playing with someone all the way through, the game is definitely much more enjoyable in co-op (much like RE5 in that regard).
As others have said, it's very different than RE5 though, I think a lot of the initial hate the game got was partially people trying to play it like RE4/RE5. It has very different mechanics and a big part of the early game is shifting to understand how these mechanics work... You can do combat roles, slide around, melee can be triggered anytime but there's now a stamina meter, with it melee is more important in this game than ever, there's a quick shot ability that depletes stamina, herbs are turned into pills that recover a bit of health for each consumed, you can now do counters if you deflect an attacking right in the few split seconds before they hit you, and more, and the game's enemies are all designed with this new combat system in mind. It's actually a very in-depth combat system, but the game doesn't do a good job at expressing it, and if you try to play it like RE4/RE5, you may find that it's not working/not really fun, as it isn't designed in that way.
As others have also mentioned, the game has some extremes differences in quality through its course. It is almost a definition of valleys and peaks. There are some moments in gameplay that are just engaging, fun, well-designed scenarios, but then there's just as many moments where it feels sloppy, uninspired, dull. It's a very uneven quality campaign, but if anything can be said it has a large amount of variety and it's also super long. There are four campaigns, each composed of entirely unique chapters. Each campaign lasts about 4-7 hours the first time usually, making the game as a whole around 20-25 hours the first time to complete all four campaigns. And there is incentive to play through it more than once, with you weapons obtained, the chapters have some big differences depending which co-op character you play as. You also can unlock unlimited ammo & purchase and upgrade skills, and there a throw-away but sort of fun monster invasion mode (where you can invade other peoples games controlling monsters), and Mercenaries in RE6 is absolutely fantastic, the combat mechanics really shine in Mercs (Mercs No Mercy on PC especially).
There are only three other things I'll add from other people's notices. Firstly, two good things, I think the monsters in RE6 are very under-appreciated. A lot of them look pretty cool/creepy, there's a lot of them, all with their own unique strategies to face them, and the sheer number and variety in them helps to keep the combat exciting. I think the monsters of RE6 don't get mentioned enough with the game, as they do have some twists on classic RE-monster types and fairly original concoctions in the fray as well, and some of the monster designs are well-done. Secondly, the game isn't a horror game, but I'd say it has more atmosphere than RE5, not in a scary way but I feel there are certain segments that resonate more ambiance and tone more than RE5 manages (less than RE4 and older games though).Now a negative I don't see many bring up either; the monsters and mini-bosses are cool, but I actually think this game has worse bosses than RE4, RE5, and Revelations 1 & 2. A lot of the bosses are more spectacle than anything in-depth. There's a few good ones sprinkled in, but I found the bosses as a whole just not up to par with series standards, and most really don't take advantage of the game's mechanics like the regular enemies do (I think Jake's final boss may be the only boss in the game that feels like it's designed around the game's combat), and many of them are just flashy cinematic things.
