I disagree, I don't think the new weapons affect the game that much. Especially not enough to be off putting. The only weapon that has truly been made obsolete is the shovel. The ubersaw is good, but people still use the bonesaw because it hits a lot faster and has a high crit rate, so it's good for killing people with. Like I said earlier the original weapons are better than all the new ones with a couple of exceptions.
For many of the new weapons you must approach the enemy differently to how you would the original, a few examples:
Huntsman - Closing the distance to point blank is likely to result in an arrow in your head.
Direct hit - Rocket jumping nearby is likely to result in you being blasted out of the air.
Bonk - Shooting with an explosive class at close range will simply make you lose health.
Wrangler - Better to target the engineer.
The bonesaw does not hit much faster (and the nature of melee combat makes it far less important) and has the same crit rate.
I think you underestimate many of the new weapons; you say the shortstop is worse just because it does less damage per hit than the scattergun, even though it also shoots faster, reloads faster and has better effective range. Even in cases where the original weapons are overall better, that does not make the new weapons useless, there will still be some situations where they are superior.
http://wiki.teamfortress.com/wiki/Item_drop_system
Essentially, its random as you play. However, by earning the class achievements you can unlock three weapons for that class at certain milestones, but many of the newer weapons, as well as all the hats, are only obtainable as random drops, or buying them through the new store.
Most of the new weapons can be crafted as well, though it takes quite a few drops.
