As far as I can see, here are Celebi's notable strengths as a Baton Passer:
1) 100 / 100 / 100 defenses ranks among the highest overall bulk spreads in the tier. We've all experienced battling Jirachi and Celebi in the tier enough to understand just how bulky that is in OU. It's significantly tougher than the likes of Rotom, Zapdos and even Swampert. This will really help Celebi set up despite a number of weaknesses. Scarftar is not 1hko'ing this thing no matter what.
2) 100 base speed lets it outrun and setup on a number of pokemon. Importantly, you stay ahead of Scizor easily, and with your bulk + nasty plot, you will always be taking it out with HP Fire. Also, keep in mind that Baton Pass is unaffected by Pursuit, which means Nast Plot B-Pass will always be fucking up CB ttar really badly (as long as you don't say, b-pass a heatran into Earthquake . . .).
Furthermore, I haven't done calcs but I imagine +2 Grass Knot does a real number on Flygon, which means CB/LO Flygon also cannot be so at ease, considering it could lose the speed tie.
3) Natural Cure presents opportunities to absorb Thunder Wave and make slow passes that allow your pass-targets to come in more easily (while still presenting a possible chance to sweep later on).
4) On the vast majority of teams, there is at least 1 easy set up target for Celebi-- the enemy Bulky Water. Well, unless that bulky water's name is Bounce Gyarados/Sludge-Bomb Tentacruel. Celebi will be taking fairly low damage from un-stabbed Ice Beams. Vaporeon, Suicune, Swampert and even Starmie will likely provide reasonable set-up opportunities. With their value as a tank and lack of SR weakness, most players will almost always opt to switch their bulky water immediately out of a threat with a super effective attack, which gives Celebi great opportunities to set up.
5) My guess is that Nasty Plot Celebi will have a certain degree of unpredictability. For instance, just because Celebi Nasty Plotted, is it fast and bulky? Is it fast and hard-hitting? Is it slow, bulky and hard-hitting? Is it even carrying Baton Pass (Energy/Knott, HP Fire, Earth Power would be a nasty surprise for Heatran)? Add potential for Thunderwave, Screens and Recover in the fray and you have an unpredictable little bastard.
Anything else I'm missing?