Other collectable card games, both online and physical, regularly issue official bans and erratas. I've never understood why Dominion doesn't do the same, though I'm sure there is an explanation somewhere. If casual players are unaware, disagree with a ban or errata, then no harm done, they can ignore it. For more serious players and tournaments, and online where such changes are easy, I think the game would be much better for some minor changes.
The online game has to match the physical. We can't go into people's houses and change their physical cards, and don't want to generate confusion and arguments when people are used to different versions of a card. And you can always just not play with a card you don't like. So we avoid errata. Dominion 2E adds "you may" to three cards so there's no "are they cheating" issue, Intrigue 2E drops the Masquerade pin, Outpost technically has multiple extra turns from different sources play out differently, and then Possession takes debt into account. While we went through and fixed wordings on cards for all pre-Empires expansions, the other changes were non-functional.
While I did change Possession and Masquerade, the chance of me changing another card for power level reasons is extremely low. If you don't enjoy the power level of a particular card, well, don't play with it, that's my advice. If I somehow end up changing the cards for more expansions ala Dominion and Intrigue (and I sure hope I don't), I will replace cards I think are so strong as to make the game bad, not errata them. Masquerade got by with errata because the change almost never matters.
(1) Possession
I would not do Possession today, it messes with the rules too much.
(2) Tournament
My only issues with Tournament are that it's extremely wordy, and I blew it on Bag of Gold and to a lesser degree Diadem. Tournament makes the better player more likely to win, not less. See:
http://forum.dominionstrategy.com/index.php?topic=2798.msg47781#msg47781(3) Ill-Gotten Gains
Here for simplicity I wish it were just, $6, worth $2, when-gain hands out Curses.
(4) Cultist
I haven't put in the games to get sick of Cultist. I think I have to buy Cultist but that I don't need to go for chaining them. However I recognize that a lot of people are unhappy with it, and like the suggestion someone made, that you either chain or Ruin, not both.
(5) Sauna / Avanto
It's a promo. The whole concept of promos is awful. Part of it is either wasting time on these cards, or accepting that they won't be as balanced. I don't know if I mind Sauna / Avanto, I enjoyed it when playtesting it, but I know people are singling it out as strong.
Given the number of cards in Dominion now, I think it's pretty good to have only 5 cards that need changing. But why not make the game better if you can?
The way to make the online game better is to let each player ban 5 cards from their games; those cards are never randomly picked (but can be forced into a game), even for rated games. I advocate this feature! I don't need anyone playing with e.g. Tournament who doesn't want to.
IRL just don't play with the cards you don't like. There are always going to be people who don't like whatever card; I can't just make the expansions full of bland unobjectionable things. On your list, Tournament is a stand-out card, a card some people have lots of fun with.