Technically, it's possible to play entirely on one world. Most of the replaced content is very similar to the original content - Tin is only a tiny bit better than Copper, the Crimson is very similar to Corruption, etc. So long as you're lucky enough to get a pyramid with a Sandstorm in a Bottle, you can play the entire game on one world without missing out on anything major.
This isn't entirely true. First of all, there is only one creature in the game that drops vitamins; one of the ingredients used in making an ankh shield. That creature is the corruptor, so you CANNOT make an ankh or ankh shield without at least some corruption in your world (Note: Regardless of whether or not your world has corruption, the Dryad will still sell corrupt seeds during blood moons. You can then seed a patch of corruption in a crimson world and farm it for vitamins and such).
And while that does have a workaround, there is no workaround for the hardmode ore differences, which are more significant than the pre-hardmode ores. Palladium, Orichalcum and Titanium all have special set bonuses that are quite unlike the ones provided by the pre-1.2 ores. In general, they provide higher defense and lower weapon buffs while also having their special abilities (things like greatly increasing health regen while fighting, calling down damagine flower petals whenever you hit an enemy, or allowing for a one-use with cooldown 100% dodge after successfully striking an enemy).
Also, you have the differences between shadow orb and demon heart drops, which can be fairly significant all depending (for instance, the shadow orb magic drop, the Vilethorn, is fantastic. The demon heart magic drop, the Crimson Rod, is utterly useless).
In case anyone hasn't noticed, I'm having a bugger of a time trying to decide which articles should receive capitalized names and which ones shouldn't...