IanB said:
The combination of reach with 2 steps means that a character can start out right next to someone, full attack them, get an attack from their partial action, and end up 10 feet away, forcing yet another attack when the foe tries to close the next round. (Unless they too are hasted...
).
I must be missing something in your example.
Full Round Attack, Partial Attack, move 5 feet. 10 feet away as you state (this must be a Spiked Chain or the Ranger is polymorphed into a large creature since you are attacking from 5 feet).
The foe then moves 5 feet closer, does his full round attack. You get no AoO since he only moved 5 feet.
With a normal reach weapon, you could be 5 feet away, back away 5 feet and get both your full round and your partial, then move another 5 feet away.
You would be 15 feet away and if he closes, you would get an AoO.
But, I see little problem with this. You are hasted. He is not. Even without this rule and with or without reach weapons, you could disarm him or trip him with your partial (moving 5 feet away first if you have a reach weapon), then you could full round attack him.
His options are extremely limited at this point. Not much different. Even in your scenario, what says that he has to come after you. A lot of high level opponents have either spells or missile weapons or reach of their own (you are 15 feet away, he moves in 5 and full round attacks and you get no AoO). A lot of opponents could just fall back (you being 15 feet away, you could not do anything about it) and pelt you from a distance.
The point is that yes, what you suggest could be a good tactic. But, at high level, there are a boatload of good tactics. And, a lot of creatures may or may not be hasted at high level. So, what you suggest is not any more impressive than a lot of other combat tactics that can be done at higher levels. And, you need a spell or item to do it (just like a lot of high level tactics need a spell or item or feat to do it). Compare Haste with your tactic to Improved Invisibility (one level higher) and any combination of weapons/feats. Improved Invisibility generally still kicks Haste's butt in combat until your opponents can see or dispel invisibility, and at those levels, they can usually also dispel your haste.
And, personally, I think that reach weapons could use a little beefing up as it is. Having one more tactic for them is great.
All in all, this can influence the game. But, why should the spell casters be the ones to gain so much from Haste (doubling their spells from 2E) and the combatant types lose so much (they doubled their attack rates and movement in 2E, now they gain 1 attack if they decide to not take the extra movement)?
Balance-wise, this is not really that overwhelming and I still think that you are over emphasizing the power of it. There are a lot of tactics and spells that can overcome this one.