My long, opinion answers.
1. In my opinion, the plot was pretty bad. If you recall the discussion, Peach was "surprised" that Bowser Jr. was her son. What is that? She doesn't know whether or not she and Bowser had a kid? I think you'd know. It could've been a whole lot better if Shadow Mario weren't Bowser Jr., but Mario's counterpart from a parallel dimension, with Corona Mountain as the portal to that dimension, and you even get to see Shadow Peach, maybe have Shadow Mario work for Shadow Bowser... I'm making this all up as I type it, you know.
2. Why would anyone hate such a great game? It's DS seller #1! It's one of the best games ever, remade after 8 years, with characters we made so many rumors looking for, a slightly better plotline if you ask me, more stuff to do, the best minigames I've ever seen (not like plain boring minigames you'd expect to be added to an adventure). SM64DS gets an A+.
3. Yep, too many. Except Mario Kart, that can stay. They'd probably be better if they were all online.
4. Slightly?! WAAAAY overused. I'm waiting for Petey Piranha to become one of your freakin' partners in the next RPG!!! It was a bit disappointing that the trophy scene in MKDD had to be SMSu themed again.
5. I don't "hate" Mario Party. I just don't see the need to own more than one--pick the one with your favorite minigames/boards and that should satisfy you. It IS an awesome multiplayer game. When Bowser's head explodes in Bowser's Bigger Blast in MP4, it's a pretty hilarious visual. Mario Party is altogether a great series... if it were less of a series.