It would work better as an IRA/Roth IRA than as a purely taxable account because the income generated is treated as ordinary income by the IRS (not as capital gains), so you pay your marginal rate on it.
As far as active/passive, it's definitely on the active side of the spectrum, particularly if you are the sort that wishes to set all the criteria by which you choose to approve loans in the screening tool for investment. If you just have the site generate a portfolio for you, that will go a lot faster - but has the downside of many notes not ultimately being funded (some of them will ultimately be declined by LC). If you pick the criteria, you can have it only display those that have verified income/approval by LC to reduce/eliminate this problem, but it will also significantly reduce the number of notes.
Once you've committed your money, it gets easier; you just log in periodically to reinvest whatever cash accumulates in the account. How often you need to do this depends on what you've invested. I have a little over $20k in there, so I log in once every week or two and put the $200 or so that has built up into new loans. Since you're getting interest+principal, the cash flow can be quite high on larger balances, so you need to be aware of this to keep the idle cash problem to a minimum.
As far as returns go, mine is holding just above 12%, and I've been on the site for approximately 2 years. Default rate is 2% so far, but if all my 31-120 day late people default, that rate will climb to 3.5%. All in all, it's not a bad way to invest - it's basically a higher risk version of bond investment. You just have to keep in mind some percentage of your loans will go bust, and that your ROI will start high and slowly drop as this happens. I expect mine to stabilize somewhere just under 10% over the next year or two; accounting for my marginal tax rate, my effective return will be somewhere around 7% - not too shabby.