"Bland": I think most of the time this complaint arises because the story beats/character motivations are unoriginal; just tropes stapled into the story without any further thought. The action movie hero is a man? 9 times out of 10, he wants to save/avenge his wife/girlfriend (or recently, daughter). We've all seen that before...
You can avoid this by simply putting some additional thought into these story elements and making them more unique to the circumstances. For example, in the first John Wick movie the hero wants to avenge his dog. ^^; It's a very small change to the trope at the end of the day, but the mere fact that it's something significantly different makes it memorable, and prompts the audience to think about what kind of character this is, and to stay to learn more.
"Predictable": As others have said, predictability isn't necessarily a bad thing. If you put too much focus on "subverting expectations", eventually you end up simply subverting the expectation that you have a satisfying story to tell. ^^;
When people complain about a story being "predictable", though, I think they mean that when a problem arises, the writer chooses to resolve it by making the 'safest' choice. The one that keeps the tone and stakes of the story almost the same, the one that ruffles the fewest feathers and invites the smallest consequences. The more accurate complaint would be "predictable AND boring".
Like, since everyone's talking about character deaths: there's nothing actually wrong with preventing your main characters from dying; it's a very normal and sensible way to tell a story, and in good stories the audience doesn't even notice. I've never seen anyone complain about how many Lord of the Rings protags manage to survive to the end, against honestly staggering odds...
Anywhom: it's the WAY you have your protagonists survive that will get your audience to appreciate it regardless. If they have to make sacrifices; losing allies and advantages and motivations along the way, it will be clear that your story still has stakes. We know the MC will survive...but how?? They may not be the same after this story ends...they may have to give up their peace of mind, their stability, their sanity. They need to keep moving forward not merely because the plot says so, but because every step they take changes them so drastically that moving backwards isn't possible. <--This is interesting to watch.
On the other hand, if they survive simply because things don't happen to them and don't affect them in any meaningful way, that's when people start saying it's "predictable" that they're still alive. A bomb devastates the MC's hometown...but all their friends are okay and they still have all their important belongings and they stop thinking about the event pretty quickly. The MC gets fatally wounded...but the magic healer character heals them and they come back exactly the same as before, with no handicaps or weaknesses or traumas. <--This is not interesting to watch. It's predictable AND boring.
"Forgettable": There's a lot of ways for this to happen, and 'forgettable' is a pretty subjective term. What can be forgettable for one viewer can be unforgettable for another...but I think the best way to generally prevent this from happening is to always give 110% where it counts. Pour your heart and soul into your selling points; figure out what your audience will expect the most, and make sure you deliver.
Because they tend to start forgetting the moment they realize their investment won't be rewarded.
I've heard a lot of people mention how strange it is that Game of Thrones was a pop culture juggernaut in its heyday, and then just blipped off the face of the earth right after the finale. Like, no one talks about it casually or seems to care much about it anymore...and if I had to guess, it's because those final seasons were so convoluted and desperate to "subvert expectations" that they managed to destroy the investment of most of their audience.
People won't remember things when it's not worth their time, when you intentionally take events they're looking forward to and characters they love and throw them out of the plot in an effort to "shock" them. Or, y'know, if you're just paying so little attention to your own story that you ignore what you're clearly setting up. It's disorienting in a way, like you've handed them the beginning of one story and the end of another. And when your audience feels confused rather than entertained, they will forget.