Celebi should definitely be on this list. I'll ignore for a second the Baton Passing capabilities that this Pokemon possesses (the ability to pass 101 HP Substitutes, Calm Mind, and Swords Dance), because it seems like Baton Passing threats like Ninjask and Jolteon are being questioned in the first place.
With Calm Mind, Recover, and 100 Base Stats across the board, Celebi is naturally difficult to take down. The only real physical threats to Celebi being the Bug type (Heracross's Megahorn and Scizor's X-Scissor come to mind) and the Dark type (Tyranitar's Crunch). Celebi can load up on the physical side defensively, and then boost its Special Defense using Calm Mind and recover off any residual damage.
Celebi@leftovers
-Calm Mind
-Recover
-Grass Knot
-Psychic / HP Fire
evs- 236 HP / 136 Def / 136 Spe
Those evs get you 400 HP, 297 Def, 270 Spe.
How does this apply to Celebi's threat offensively, however? Well look at what Celebi really fears defensively...Heracross, Tyranitar, and Scizor. After a Calm Mind, Psychic from Celebi OHKOs Most Heracrosses, and HP Fire from Celebi OHKOs Most Scizors. After a Calm Mind, Celebi's Grass Knot is also a guaranteed 2HKO on almost all versions of CB Tyranitar, while a max attack, Adamant Choice Band Tyranitar only has a minimal chance of OHKOing Celebi back with Crunch.
Those are just specific dangers to the Pokemon itself. What about generally speaking? Well, with Recover, Blissey certainly isn't a counter to Celebi. When the ultimate Special Wall is helpless against a special attacking threat, that threat needs to be respected. The only way, honestly, of dealing with it is having a faster, strong hitting STAB SE attack coming it's way, preferably from the Special side. The Physical "counters", Heracross (needs a Scarf, which is losing popularity to Band), Scizor (always slower, and fears HP Fire on the CM set), and Tyranitar (Grass Knot) all cannot be considered counters. Even from the Special side, if say, Gengar or Infernape, both with STAB SE moves to utilize against Celebi, come in on a Calm Mind, neither will OHKO and both will be OHKO'd.
Why does it deserve to be on the threat list? Because if you mistakingly allow it 1 or 2 Calm Minds, you're essentially forced to either have a particular counter in hand (like Yanmega's Bug Buzz or Heatran's Fire Blast), or sacrifice a Pokemon using Explosion or Destiny Bond.
I am of the opinion that all bulky Pokemon with stat increasing moves should be considered for this list, and when you add decent Speed with a 50% Recovery move, I feel it is a necessity.
New players should be aware of the threat Celebi poses. And this, of course, is simply viewing Celebi in a singular sense, and not the team threat it poses by being one of the most efficient Baton Passers.