In fact, I want to say that there are people like me who have eliminated saboteur from the game, and I also read that it is banned in some tournaments.
Irrelevant. Tournaments ban things all the time for no reason other than it might give a SLIGHT advantage to the player. Even if whatever was banned had TONS of play testing (like Dominion), people will still ban stuff because they deem it "unfair". Personal opinion doesn't change the fact that Saboteur has been play tested a lot and is deemed by DXV to be a balanced card.
so I think that is not a good card, maybe it is not broken, but it certainly is not good.
Again, it's a matter of perspective. Do you want to get slightly ahead and Saboteur is on the board to destroy your opponents deck? If so, then go for it. If you think you can find a better combo, use that instead. There's really no "bad" or "good" cards, just how you use them. Saboteur is a pure attack card. It can make you jump ahead by trashing their Province or it will trash their actions and mess up their strategy. If you don't like it, fine, but it's no reason to remove it from the game. Someone else might like it and use it.
About seaside, I do not like the mechanism duration because I do not like to remember the bonus that I will have next turn. I like the normal mechanical where each round is played and then cleaning it.
If you read the rules to Seaside, it's really not that difficult of a concept. You leave the duration card out so you
don't have to remember it from memory. That's how they work.
It also looks like you are disliking things just because it's a new mechanic. It's typically the mindset of someone who hasn't played many games and doesn't grasp the multitude of things Dominion showcases. Most beginners don't like trashing because they don't want to lose cards. However, if you take out all the junk in your deck, you are most likely to win (Edge Case: You Open Death Cart/Chapel, Chapel everything besides Death Cart, then use Death Cart on Chapel, don't buy anything, play Death cart again, then trash Death Cart because you didn't trash an action). You say you like cleaning everything up because it's "normal". Duration Cards don't change that, you just clean up everything minus the duration cards for that turn. Then on the next turn you clean it up.
I do not like alchemy because of potions. I do not like cornucopia because the prize cards come out only if there is a tournament, otherwise adds little. I like interaction, and I'm looking for attack card but not frustrating as saboteur.
It's becoming increasingly more and more obvious that whenever you see something new Dominion offers you reject it immediately. Also, you are making very big generalizations that already put you in the mood to hate something. It's like a kid who gets fed a hamburger. The kid goes CRAZY for the hamburger and loves it. Then when you try to give the kid something else like a salad, the kid just says "Yeah, but it doesn't have meat. I like the meat the hamburger offers". OF COURSE the first thing you try is going to appear perfect. That doesn't mean everything that comes afterward is flawed and "adds little". We don't know what you like. We are showing you all the different flavors and dishes yet you cling onto the original burger you were offered with Dominion and deny everything else even though you haven't tried the other meals.
Someone said it earlier about how Dominion offers some complex mechanics in the base set and the following expansions; if you don't like what Dominion has to offer to change up the game so it isn't the same game every time, then maybe Dominion isn't the game for you.
Again, it's best that you play a dozen or so games with each expansion and have it influence your decision for later games. If I play several games of Dominion: Intrigue and find that I personally don't like Secret Chamber, it will have a small influence over me for randomizing games. I'll take a look at Secret Chamber and say "Does it look like it will offer something in this game?" If it does, even in the slightest, I'll keep it in the game. If it doesn't, I'll look to see if I could maybe replace that one card with another. I DON'T remove it from the game and never consider it ever again, which is what you seem to suggest every time you dislike a new card or a new expansion that you don't have.
tl;dr You should keep all the cards and have experience, not first impressions like "Sabotuer is bad. It's broken. I don't like it.", dictate which cards to use. This topic is a bit silly now.