As far as the levitical laws, for the most part we actually follow them today.
They described crop rotation, bathing, illness identification and structural fumigation. All these things we still do today.
Most people concentrate on the dietary rules but why are they bad? Most of the animals forbidden were generally poor food or even deadly. Pork for instance is still a meat that has a disease risk and still kills people every year.
How about the marriage laws? Remember, people often died young and a surviving wife's only chance of living was to remarry and so she was married to the closest male relative rather than a stranger. In this scenario people need clear rules on who can marry whom without inbreeding too badly.
How about the homosexuality... If you actually read the bible, homosexuality was only mentioned about a half-dozen times. The Israelites were forbidden to use sex, homo or hetero, in the worship of the lord and they were not allowed to use rape, homo or hetero, as a means of interrogation.
There are only a few parts of the old-testament law that I don't really get. The first is to not "Stew a lamb in it's mothers milk." I guess maybe they thought it was cruel or disgusting or something. The next is "Only wear clothes of the same material." I have to assume this was to keep the clothes from wearing out since different fabrics wear at different rates which tends to shorten the life of the garment over either material alone. But both of these are guesses on my part and really minor concerns.
As far as old vs new, there really isn't that much difference. You car comparing writings that are at LEAST 2000 years apart. When you compare the new testament to Malachi, the youngest old testament book, you find far fewer differences even though these are almost 500 years apart. There were other texts that did not make it into the bible that actually show a fairly smooth transition from genesis to Matthew.
As far as Paul vs Mark, there actually aren't many differences. Remember, Mark was reporting history (he even describes himself as a reporter) while Paul was taking the principles and creating an operating organization. There are aspects in real life applications that Jesus did not go into great detail on and Paul filled in those details.
Actually I really respect Paul because he taught that 'Jesus said this, I believe this, but it would be ok to do that.'
Excluding the Unitary Universalists who are not and don't claim to be Christian, what major differences are there between the various mainstream sects of Christianity? Excluding of course who it the earthly head of the church.