I already left a non-spoiler comment on the main page. But i just feel i have to elaborate. I'll mark when the spoilers start.

I read the Law of Ueki all in 1 go, and i just loved it. It started out as your standard shounen fare, but eventually shown me some original ideas. One thing that I really loved was how it waited practically till the last battle to do the "Power level vs Power Level" fight. The rest of all the fights was always Brain + Brawn(Power). Kateikyoushi Hitman Reborn started with the "Creative" thinking fights then enventually became a "Powering up" kinda manga. Which is the standard shounen fare along with Bleach and Naruto alike.

It also kept the same amount of comedy from the beginning to the end. Which alot of Shounen series either fail to do or just decide to "Evolve" from. Hitman Reborn and Naruto had alot of comedy at the beginning, but after the Time skip(See what i did thar?) went into a more serious flavor. Which is fine, but then a new problem arises when the author actually tries to bring back some comedy into his series then. Now it feels forced.

You grow to love everyone in the first series. Mori, Rinko, Sano, Hideyoshi and Ueki all become 1 big family, and lets not forget Tenko either. Also, the 1st series was just the right amount of length and it actually wrapped itself quite nicely. It was truely a "Happily ever after" ending. Length is a big issue here because as longer shounen series has shown us. The only way to go is "Up". Lets not forget what happened to Dragonball Z which takes it to ridiculous levels. Which is where the whole "Lawl power level" jokes come from. People began making fun of Naruto and Bleach for getting into "Dragonball territory".

Spoilers are going to start appearing here.
As far as when the first series ended. Not everyone in the team actually got their "Level 2" powers. So if a sequal were to ever come out, that could have been a good time to upgrade everyone else. Well we got our sequal. But that was just a kick in the balls, and i'm still hurting. The entire Plus series is one big giant reset button. by the time we get our plot twist in the middle of Volume 5, we get the end. Half a volume is NOT enough time to tie up the plot twist. And there was a 3 month time skip in that half? You can't tell me this wasnt rushed.

Did the manga-ka just decide "I dont like this anymore."? Let me show you my thought process as the story went in.
Volume 1
Awesome!~Wha'ts that?~What??？??？??
I don't like how everyone has their memories erased. What do you mean Ueki has no powers?!
Volume 2-3-4
Oh....~yes....YESSSS I like where this is going!
Volume 5
Tetsamaru: YEAAA!!!
??:Hello
Tetsamaru:Who are you?
??: I'm random plot twist.
Tetsamaru: Oh, well, hel-
Plot Twist: KICK IN THE BALLS!!!
Tetsamaru: OH SWEET JESUS!!!! THE PAIN!!!! WHY MAN! WHYYYYY!?!?!
??2:Hello
Tetsamaru: Who are you.....?owwww
??2: I'm the ending.
Tetsamaru: Huh? Alre-
Ending: KICK IN THE BALLS!!!!
Tetsamaru: #$@#%^#%&%$^$

It just seems rather than go on a Hiatus because of lack of inspiration or time constraints or budget. The manga-ka just decided, "Meh....this sucks now". and ended everything in 1 swift blow. Now let me be clear, i DID NOT like how Plus first started out. As it pretty much undid everything that happened in the 1st series. We didnt even get to see Inumaru or Kobayashi sensei. BUT, after volume 3, i was honestly starting to like it again. I fell in love with the 1st series right from volume 1. I started really liking Kateikyoushi Hitman reborn after volume 4. So sometimes, a series can have a slow start.

I almost want to say Plus forgets the existance of the first series but Ueki at least showed us he still remembers everyone and reminds us that that's the sole reason for getting his new powers to fight. I feel like i should make my own doujin on how plus "Should have" been.

In the end. If you really liked the first series. Do yourself a favor and don't read Plus. it adds NOTHING to the first series. No extra character development, no more backround info, no nothing. It not only fails as a "Sequal" but it also fails as a stand alone series.