It depends on the practice area and the specific firm.
The boring/mind-numbing parts are document review (in litigation) and due-diligence (in corporate). These both just involve reading over documents and flagging them if they seem important. In the case of doc review, you might literally be reading thousands of random emails and clinking "relevant" or "not relevant" on a computer screen.
However, most associates will also be doing some memo writing even at the beginning. In a specialty area like tax you may be doing research and writing almost exclusively.
The non-litigation side of things can be finance related, but you do not need a finance background. While you need to understand the basics of finance, you will not be doing any of the math. It's rare that you would need to know much about accounting. The only exception would be tax. Even then, it's rare that you would need to actually crunch the numbers to any significant degree- you would just need to understand tax accounting concepts.
There are folks who genuinely enjoy it. For the most part, it's the junior stuff that really blows. I doubt many people are ecstatic about doc review, but those who fit in well with the biglaw lifestyle move past that stage. The soul crushing part is generally not the work itself, but the forfeiture of your freedom. Your blackberry could hail you back into the office at 9:00pm on a Friday night. People start resenting that after so many canceled social engagements.
Biglaw vs. ibanking probably depends on your personality. I don't think it's possible to generalize.
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