Typically, an alpha strike is thought of as firing ALL your weapons at once. (check the urban dictionary)
Energy based mechs tend to have the highest alphas (best damage per ton), but pay for it with high heat (oft risking shutdown).
Alpha strikes, theoretically, are for when you face likely quick death and need to do as much damage as you can before you die. In practice, many players/mechs simply use alphas constantly, particularly those that boat energy weapons. For general use, it was (mistakenly?) thought that you would be firing only portions of your weaponry at a time. Instead, alpha-ing seems the norm.
Heatsink buffs (particularly for clan) make alphas more viable, rather than less.
Ghost heat was added to discourage an alpha by making shutdowns and the possibility of heat damage more likely.
Chain fire, and weapon grouping to some degree, is designed, kinda, to allow people to rapidly fire large numbers of weapons without inflicting quite the heat burden from simultaneous fire. Both effectively lower alphas.
Energy Draw (as I understand it, I never played on the pts), was basically set up as a capacitor system, only so much power could be delivered at once, so it limited how many weapons could be fired in a given time period (kind of an alternate and mandatory chain fire). Again, the idea was to rein in the alpha strike, because you simply couldn't fire that many weapons at once. People apparently hated it (I suspect for one or two reasons, possibly both: one, poor implementation, possibly unreasonably limiting; two, it actually did its job and prevented high alphas, which caused all sorts of mental strife and mandated {unwanted} play style changes)
One of the subcurrent concepts here is that people seem to be unwilling or unable to restrict their weapon usage to mellow out heat and burst/alpha voluntarily. So PGI tries to do it artificially.
High alphas (or just say initial burst damage) have the effect of decreasing the survival time of every type of mech, but particularly those of lower weight. Especially with focus fire, high burst damage can down any size mech with surprising speed.
Results? Light/medium mechs are played less often. Quirks to improve durability (armor/structure buffs) are rampant. Boating to allow simple (and massed) weapon groups is common (range specialists, rather than a mixed load). Ghost heat. Weapon nerfs. Forum threads to discuss various related topics (like this one
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I personally would prefer more of a tactical match, less of a rapid delivery slugfest, so I favor limiting alphas somehow.