The story, at least for me, was interesting up until they revealed Big Mouse. Wanting to know who Big Mouse is, was the driving force for me to eagerly wanna watch the next episode. Still interesting to watch until the end though.
I think the ending, although disappointing, is fair enough to wrap up the story. The whole disappointing feel for the viewers may be because they didn't get the justice by law despite Chang Ho wanting to bring Do Ha down without getting his hands dirty and Mi Ho's effort felt like it all went down the drain.
Best girl didn't even witness Do Ha's fall (like literally). I can understand that both Mi Ho and Do Ha's ending was inevitable though; without it Chang Ho wouldn't continue being Big Mouse.

About character development,
Chang Ho's felt like rushed? I mean he's an incapable lawyer so I assume that he's not that brainy but suddenly he can pull off being Big Mouse. Is it survival instinct?
Ju Hee's last minute development was already foreseen. Seeing that she's not totally evil, it's easy to read that she'll end up betraying Do Ha.
Yoon Gab is a bit questionable. I get that he's loyal to No Park but he seems like he wanted to be no.1 after his death instead of a newbie coming out of nowhere and suddenly claiming the throne which is fair because he's been a long time member and is in top3. I thought he'll end up questioning why Chang Ho continued being Big Mouse. Last ep was rushed though so conflict like this would prolong the story. Let's just say that he saw that Chang Ho is capable hence his approval.
I was waiting for Prosecutor Joong Rak to betray Ji Hoon and stick to whom he thinks got the power but surprisingly, he remains loyal to Ji Hoon.
Gong Ji Hoon is the main surprise for me here, he's a deranged, rich guy and I thought he is the worst character here. Never thought the twist his character will have. Who would have thought him and Do Ha would swap sides. He is the true winner here coz he didn't lose anything.
Jerry, on the other hand, got the character development but went from interesting character to unnoticeable side character when he became Big Mouse' lackey.

Still many questions left like that pyschopath, serial murderer. Kwang Yeon deserves his truth to be known, for me, his death is more tragic than Mi Ho's. He didn't get to save his mom despite being branded a psychopath until his last breath.
Hye Jin was forgotten, her husband deserves death penalty.
Did I miss it or they didn't clear Chang Ho's name about the 5 billion won bribe?
Didn't they question Do Ha's sudden death?
Even if Do Ha's dead, they should have revealed his true identity which is the reason why he killed the chairman. Old man doesn't deserve justice but I need Do Ha's true colors known to the public. Even the chairman's butler needs to be put on jail for conspiring with Do Ha.
Although many are saying that they wanted Season 2, that would be a bit of a stretch and might ruin the story.

Overall, the last episode was underwhelming but the whole series was a fun ride!