It's slow in it's psychic form, and fast in it's fighting form.
Base 90 speed is not slow. It ties with Lucario and beats Rotom-W, and no one calls either of them slow. In fact, it's slightly above average, seeing as how the average base speed of OU is about 83 and it outspeeds over half of the tier.
Since you are required to use a move to transform first,
And SD Virizion really needs to use SD to become dangerous, but no one complains about him. Besides, I've already explained how Relic Song can be a useful move in and of itself, even aside from the fact that it transforms Meloetta.
can't use a choice item while in fighting form
And? Choice Specs is not a good idea on a Pokemon with only 77 base SpA and Choice Scarf is a waste on a Pokemon that already has an incredible base 128 speed stat. All it's really missing out on is the Choice Band, and even then Meloetta is far better off throwing around Close Combats and blowing past Skarmory with Thunder, Gliscor with HP Ice, etc.
and have a sub-par typing in normal, its simply going to be border-line and a gimmick.
Normal/Psychic isn't a great typing, but then again, since when has pure Psychic been a coveted typing? Sure, you lose the Fighting resistance, but the Ghost immunity means you have one less weakness than pure Psychic types.
Again, yes, a high powered move off of a high attack/special stat doesn't make an OU pokemon alone, if it did, Typhlosion would be OU immediately as well as Sceptile with leaf storm.
That was never the argument. No one said that Meloetta will be OU just because it has powerful attacks and powerful offensive stats.
But they aren't know why? Because they are predictable.
No, Typhlosion isn't OU because it has a SR weakness, a shallow movepool, and fairly average stats. It also doesn't have much to distinguish itself from Infernape besides access to Eruption. Sceptile isn't OU because of its frailty, average power, and the fact that other offensive Grass types like Breloom and Virizion have qualities that make them more desirable in OU. In fact, Sceptile's ability to run a special sweeper and physical SD sweeper sets makes it rather unpredictable.
Meloetta is predictable and a wasted slot on most teams.
How, pray tell, is a Pokemon that can go from a special tank to a lightning-quick physical sweeper with a virtual flip of a switch predictable? Now, you want to know who's predictable? Skarmory. It literally runs the same set almost every time, with the only major difference being physically or specially defensive. Guess what tier he's been in since Gen 2.
Reuniclus does what the Psychic form does so much better and with better typing,
I don't see how, to be honest. Not only is Reuniclus slower than Christmas, it's less bulky and less powerful. It also somewhat struggles to get past Tyranitar since it has low speed and relies on a shaky Focus Blast, while Meloetta breezes by with Close Combat. "Better typing" is also debatable, since adding Normal to Psychic typing trades a resistance for an immunity and loses a weakness at the same time. Not that mono-Psychic is a great typing to begin with.
and the fighting form can't do what scarf terakion does or well, what any fighting pokemon can do due to the fact you have to waste a turn just to get this thing into the fighting form.
I'm starting to get the feeling that you've been using Meloetta wrong. Scarf Terrakion is a revenge killer. Meloetta's P. forme is built to sweep mid to late game. They're two different Pokemon with two different roles. Not only that, but Scarf Terrakion lacks the power that Meloetta with a Life Orb has, cannot switch moves, and struggles to sweep teams when it's trapped into one move.
Use this thing, you'll fail when someone predicts your relic song to turn into the fighting form to sweep then kill you off with a scarfed revenge killer.
Or they send in, say, Scarf Terrakion, only to meet a STAB Psychic coming off a huge SpA stat.
Unless you can sell me on the Psychic/Normal form doing something productive, this thing will not be that useful with Alakazam, Lati@s, Reuniclus, Metagross, Jirachi and a host of other psychic mon's doing what this thing does better.
What exactly do you think Meloetta does that those things can do better? Meloetta-A is a special tank unlike any other OU Pokemon right now. The only other OU Psychic types that even come close as special tanks are Reuniclus and Latias. Neither of them has quite the power or special bulk that Meloetta has, and each has other problems that keep it from outclassing Meloetta. That's what Meloetta does. It's a Psychic type special tank, and one that can get past Tyranitar without leaning on an unreliable Focus Blast or a puny HP Fighting.
The biggest issue I think with Meloetta's movepool is lack of alternative to Close Combat.
So now Meloetta needs an alternative to the best physical Fighting attack in the game?
Another issue is faces is that Relic Song isn't even a good attack. 75 base power is atrocious.
Drain Punch and Giga Drain are both seen as useful moves on a variety of Pokemon, the main STAB move of some, and they also have base 75 power. Hidden Power only gets up to 70 and it's used on a large variety of Pokemon for coverage. Base 75 is fairly low, but definitely not "atrocious."
128 SP.Atk is above average in OU,
Yeah, and Mt. Everest is a pretty big hill.
but it cannot make up for the 102 base power attack that doesn't have anything it's super effective against. It's movepool is decent special-wise, but it's physical movepool is limited to Stone Edge, Quick Attack, Close Combat and U-turn I guess. Shadow Claw is viable too, the problem is that if a Ghost is out then Relic song cannot be used anyway to turn into fighting form.
But what Ghosts are eager to switch into Meloetta? Gengar is scared to death of a STAB Psychic move, and Jellicent hates Thunder to death.
Serene Grace Psychic form is the only way this thing will probably ever be used.
A P. forme mixed sweeper set works just fine. All it really needs is Close Combat and a couple of utility moves to get past Pokemon that can take said Close Combat.
Thunder, Psychic, Shadowball, Focus Blast as a movepool maybe /w Scarf? Coming in on nasty ghost moves and special moves alike? But then again, Jirachi does this so much better due to typing and better speed.
Then don't use it as a Scarfer. The Hone Claws stallbreaker set is probably the best that Meloetta has to offer, and Sub CM lets it perform like no other Psychic type in OU can. I think the main reason these discussions about a Pokemon outclassing Meloetta come up is because people have this misconception that it has to perform exactly like the other Psychic types. People think that it has to run a moveset like Psychic/Shadow Ball/Focus Blast/filler just like Alakazam and Reuniclus, or that the P. forme has to run a full physical set just because it has high Atk. When you actually take advantage of Meloetta's niches, then you quickly find that it is capable of things that no other Psychic type can overshadow it in.
It's typing and movepool are just average and that's the problem. It's gimmick is just that, a gimmick. If they release a way to make it start off in it's Fighting form, then it has more potential but even then it's still predictable, and has bad typing and a limited movepool and useless ability. Serene Grace literally does nothing for it's Physical form... bar RAWK SMASH, but lol at rock smash.
...did you just call Serene Grace a useless ability?