Charizard is rendered useless by a lot of things, Cloyster is only a threat to teams that rely on Magneton instead of a Spinner, Houndoom covers some very specific holes in teams weak to Celebi and Jirachi, Umbreon is rarely a threat since Phazing is so common. They're not so much threats as they are pests. Moltres might belong on there, if there weren't so many bulky waters. Tauros, on the other hand, can dish out a whole lot of damage to anything, has base 110 Speed (which only Aero and Duggy beat as far as Banders go), Intimidate, giving it some decent switch ins, and the ability to hurt pretty much anything, barring Skarm/Forry. Slaking is a threat because there is pretty much nothing it can't 2HKO, with prediction ofc. Even Forretress cries at a mighty Focus Punch. Same goes with Medicham (except there actually IS nothing it can't 2HKO). The Pokemon listed as threats are threats because they can piss off just about any team, and quite easily work the match in your favour. While they differ in what roles they play in a team, in a match these pokemon tend to play the leading role in the fall of your opponent somehow. Oh, and Heracross is a monster. If you switch Dugtrio into Standard Hera you're begging for trouble. And it doesn't really care about Sand when it can nail Tyranitar with CB Megahorn/ Brick Break.