I've been playing a bunch of SMG2 all day, and I'm really into it, it's brilliant in a lot of ways. Most of it is the level design, though the new mechanics help, and I love the tiny little throwback mechanics, like the block trail, which I never expected to see in 3D. But then, like any Nintendo game, perhaps even any Japanese game, it is far from perfect. There are a few things that don't really contribute to the fun and just serve to annoy. I'd like to list a few. 
 
1. Having to wrangle Yoshi 
Now, this is kinda more a fundamental problem with Yoshi than it is a problem with SMG2 in particular. Simply put it's a pain in the ass to have to jump on Yoshi after you get hit. Always has been really if you think about it, but then it never was such a trouble as it is here. As really, several games have used that mechanic without injury. SMW first off, but also the DKC series, of which I am very fond. But you see, the problem with the One-Hit-And-You're-Off-The-Horse philosophy is that since the player can easily lose his ride at any point, it isn't really fair to require the player to have a ride through the level design. It was a challenge the DKC series overcame in later games with the introduction of transformation barrels, wherein that mechanic was thrown out the window in favor of just having life. But you see, Yoshi isn't like that, and at the same time, any time you have Yoshi you will need his skills to succeed. And this applies during fucking boss battles too, where you're on a platform full of spinies and a guy is randomly charging in with lightning, and you have to try to land on a fucking Yoshi before you can do jack shit about it. 
 
2. Return of Gearhead cleaning sequences 
If I had to say what my least favorite SMG star was, I'd have to say the one where you had to throw 5 bombs in 20 seconds with absolute precision. They've apparently not learned that that was a terrible fucking idea and increased the amount of them. I've ran into 2 so far, with a third marked on my mpa, presumably so I know to avoid it. Basically, arbitrary time limits never make anything better, especially if the end result is you have to try it again and again. 20 seconds repeated 20 times may amount to 6 minutes, but it is the worst 6 minutes you will ever have with the game. Thankfully they've been a little easier this time around but still. 
 
 3. Monkey Score Challange 
A monkey challenged me to meet an arbitrary number of points in an arbitrary amount of time doing asinine jumps, each of which gives me an arbitrary number of points. Riveting. Maybe it's better if you grew up with score mechanisms unlike myself, but I'm pretty sure that isn't the case. Really, scores aren't about competing with a preset number in the game, scores are about competing with mates. Honestly even then, the monkey's challenge is filled with so many arbitrary values that it's pointless to even consider them. It sorta reminds me of Game Room's  Challenge system, except with a soulless monkey instead of an Avatar doing the royal wave. But while the monkey is throwing out random time limits and rules with all seriousness, your actual friends are more looking to have a good time, and thus say, starting stupid deep in Yars Revenge makes perfect sense. 
  
Edit: It's like if Gamestop held a "Stupid Deep In Yars Revenge" Tournament in all seriousness. It's be fucking ridiculous.
 
4. Motion Gimmick Levels 
In the first I hated the squids and didn't really like the ball rolling bits. I can hardly believed they returned, with new variations even like the Fluzzard galaxy. Aren't there enough games with shoddy Motion Gimmicks?

That's about all I've come up with so far. Anyone else have any complaints.

I haven't found anything yet, but if I do, I'll post it here.
 
I have only gotten far enough to get to #1 and #4, but neither of those things were bad things. I loved how Yoshi behaved, made it all much frantic. And those motion control parts were some of the best parts of Mario Galaxy 1, so I am glad they returned. Also, they are not motion gimmicks, they are established ways of gameplay.

I have just spent probably a half hour trying to master a fucking 40 second minigame becuase it expects fucking perfection. Repeating shit 40 times is never fun, particularly if you have to repeat because "Your number, 590, is not higher than our arbitrary number, 600" as opposed to say, fucking up. That is the worst thing ever. I fucking hate whoever thought this was a fucking good idea.

You can also get on Yoshi by doing a swirl attack. I can't really relate with the points you raise. I've only done one monkey challenge (i don't know if there are more) and i got it after a couple of tries, once i had a good chain going it was pretty easy. The motion control levels have been really enjoyable, i'm pretty used to tilt controls with Monkey Ball so i had not problem getting used to the controls. I would quite like if Nintendo made an actual "Mario Ball" game. 
 

Nothing!

what's with all these perfect scores? admittedly i havent played it, nor do i have a/ play much wii, but when the game started to surface didnt nintendo themselves admit the game was just gonna be extra content they decided not to put in mario galaxy 1, therefore ripping off the consumer to fork out another bundle of cash for a new game title?

" I have just spent probably a half hour trying to master a fucking 40 second minigame becuase it expects fucking perfection. Repeating shit 40 times is never fun, particularly if you have to repeat because "Your number, 590, is not higher than our arbitrary number, 600" as opposed to say, fucking up. That is the worst thing ever. I fucking hate whoever thought this was a fucking good idea. "

I'm guessing you are talking about the ice skating mini game.
It's strange but I found that one and the Monkey Score Challenge to be an absolute blast. 

I agree with the while Yoshi thing, but after discovering I could spin him to get on without jumping it doesn't really bother me anymore.

But all this is easily avoided by not just going for those stars.

"what's with all these perfect scores? admittedly i havent played it, nor do i have a/ play much wii, but when the game started to surface didnt nintendo themselves admit the game was just gonna be extra content they decided not to put in mario galaxy 1, therefore ripping off the consumer to fork out another bundle of cash for a new game title?"

I'll do this by the numbers. 
 

1. It's a matter of knowing that you can shake the Wii Remote near him to get back on, but I just can't get used to that. I've always been trained to jump on Yoshi.
2. I agree that the Gearmo sections aren't the most fun, but the first one I did took only one try so... I say just practice?
3. Once again, the only Monkey challenge I did was on the first try, didn't bother me.
4. I actually really like the ball rolling parts, that's motion controls done right.

" what's with all these perfect scores? admittedly i havent played it, nor do i have a/ play much wii, but when the game started to surface didnt nintendo themselves admit the game was just gonna be extra content they decided not to put in mario galaxy 1, therefore ripping off the consumer to fork out another bundle of cash for a new game title? "

It's gotten downright sad now when people kneejerk say "motion controls are worthless" no matter WHAT is done with them. 
 
Btw I was ready to shake your hand for pointing out problems with the game, but they all seem centered around "this is too hard, or that is too hard", those are only problems that center around your own personal playing ability. 

80 stars in, I still haven't found anything to complain about, except perhaps that the secret stars are easier to find and get than the actual normal stars. But I don't think that qualifies as something that is bad.

The camera still plays tricks on me on occasion. Seems like you have to be in perfect position to grab something. 
It's a great game but it's no 10. 9.25-9.75 for sure. I wonder if you can find it in stores or is it hard to find like NSMBWii was during the last Holdiay Season.

Eh,  I've had no problems myself so far.  Granted, I'm only on world 3-ish?   But still fun.
 
The monkey is a bastard though.  But it  IS still optional and not part of the critical path.  So some of the stuff  you can completed skip if its annoying you.   And because its optional I can't really mark points against it.

I really liked the enemy stomping monkey challenges, they were just hard enough to be challenging but not too hard, and fun to do. The bowling one and the swimming one were lame though. Totally agree on the Gearmo stuff, but at least they were easier than the ones in G1, I didn't have nearly as much trouble as with those.
 
Maybe it is something about being an old school gamer but I actually do enjoy trying to beat a high score... It's better when you're trying to top yourself or a friend instead of topping the computer's arbitrary number though, like you said. I play shmups and Metal Slug games to beat my high score all the time.