Power shots are used by enemies (and, optionally, by you) once you're past the junior class, but it's really not like the GC game where they can completely ruin gameplay, for three reasons:
A) Since it's a 2D perspective, there are no issues with disorienting camera angle switches and things of that nature.
B) The animations are a lot shorter.
C) The tennis academy players aren't on the level of Mario and company, so the power shots you're using and playing against are really tame until you're near the end. And, really, even though at least half the enemy players with a power shot brag about how you'll never be able to beat them, I never lost a match due to a broken power shot, but lost plenty of times to lobs or players who just plain had superior stats compared to me.
I think that, if anything, some of the negative impressions come from four things (not counting anti-Nintendo bias as a possibility =P):
1) Certain RPG elements play a smaller role than they did in Mario Golf. For example, you can't go out and buy improved rackets like you could with golf clubs, nor are there stat boost items to find other than a few 1-up mushrooms, and there's no world map. (Which isn't to say that the game is any smaller, because it's not.)
2) You have to play some minigames to get better power shots. Personally, I didn't mind because I think they're great, but I could see how someone who really only strictly wants to play tennis might get annoyed.
3) It actually seems like you're better off only levelling three of your four main stats (there are secondary stats as well), because the stat decay is such that if you try to improve all four, your well-roundedness is going to betray you against specialists.
4) You can't play practice rounds to gain experience in the single-player mode, as far as I can tell. Not that I really needed it since the levelling curve is more than fair enough, but still, I shouldn't have to play exhibition or start a new game to play the same set of opponents again. They play up the fact that ranking matches take precedence over others by having the coach ask everyone to clear the courts, but that seems sort of hypocritical since you yourself can't play non-ranking matches anyway.
Anyway, here are the positive points for me:
-Tennis seems to be inherently more gaming-friendly than golf, due to giving the player more interaction than just waiting for a bar to move into its proper position. I had a lot more fun with this than Mario Golf, and that was a good game in its own right.
-The game feels more tightly woven-together. Rather than having a bunch of different golf clubs around with identical entryways and just kind of giving off the illusion that you're a travelling athlete, the junior and senior tennis courts are right next to each other and the varsity court is pretty close as well. I like it that way; there are fewer wasted moments.
-Like I said, the 12 minigames (counting the machines, but not counting the coaches' lessons) are extremely fun for the most part. Nintendo's pretty much become the king of Flash-style minigames, so this really isn't a surprise... I'm not going to ruin what any of them are, but basically the overall set reminds me of Track and Field, except done in modern-day style and with little anime/Mario/Donkey Kong touches everywhere, controls that actually work, and endless modes. Old-school arcade-style goodness, basically.
-There are three save files.
-Your doubles partner this time around is actually skilled and isn't designed to naturally mess up every so often or hit inaccurately; his/her AI is permanently on the highest level as far as I can tell, so the only thing that can stop him/her from being good is bad stat distribution on the part of the player.
-The characters are more memorable; the dialogue is funnier, their personalities are shown off more, and their voices are pretty easily-distinguishable from one another.
-The graphics, most notably the character models, are almost infinitely cleaner; experience working with the GBA really served Camelot well.
-There are two default difficulty levels in the single-player mode, and another unlockable one.
-Not only is the music much better than Mario Golf in my opinion, but it's less repetitive. In MG, you'd be hearing the same theme for 18 holes straight and it got somewhat boring, to me, but here there's a theme that plays whenever you're on a game point, a theme for a set point or match point, and then are the themes that play for most of a match--and lots of them, one for each two ranks of a class, meaning two themes for the junior level, two for the senior, and so on. It just makes for a nice variety. The minigames each have different music as well.