Old server chunks are static and though newer recipes etc... will work, the newer minerals and all that will not occur underground. Likewise, trees will all be oak trees, there will not be any biomes either. Not 100% sure about world height, but I assume the new world height applies to the old chunks. That's not to say you can't go plant birch trees or place stone brick stairs and all that...they just will not occur randomly.
There are major differences in the game content between Beta and current version, so playing there is sure to be a different experience.
The new zone (ie the area depicted in Corv's coordinate plot above) is 100% up to version 1.5 standards, as is the nether nearby. Heading in the direction of the old nether hubs however will bring you into "old nether" before very long, so if you're looking for, say, nether quartz or nether fortresses, you won't find either in that direction.
This post has been edited by cerveza_fiesta: 15 May 2013 - 05:18 PM