This is really mostly a law thing - banks and consulting firms, by and large, aren't partnership models any more and haven't been in a long time. Any large bank or consultancy is public now, which means they give equity to employees out of a huge pool and it has relatively little effect on the comp of their managers. That's different from a law firm, where even in fairly large ones the addition of new equity partners can have a material impact on the existing partners' comp.
As for making partner in a BigLaw firm, folks I've talked to describe it as "a longshot" to "nearly impossible." A firm like Skadden will hire 500+ new associates a year (worldwide) and name maybe 4-6 partners, not all of whom started as associates. Unlikely the chance of going from associate to partner breaks 1%.
(Updated #s after asking someone who worked for Skadden.)
McKinsey and Bain are incorporated partnerships, and BCG is still an actual partnership. Goldman Sachs, JPM, etc, are public companies, but their investment banking and trading divisions operate very differently from a typical public company. GS, for example, pays out 50% of revenues as compensation, with MDs and PMDs (partner managing director) sharing from a huge profit pool. Internally they function very much like partnerships.
As for the chances of making partner, it depends heavily on the firm and the year. Skadden is something of an outlier[1]. The other big NYC firms bring aboard about ~100-120 associates per year, and promote between 5-15 depending on the economy. The last few years it's been around 5 and during the boom it was around 15. At medium-sized firms outside NYC, the %-age is more like 20-35%, though profits-per-partner at those firms are more like $300-500k/year, versus $2m+ at Skadden.
[1] Though your numbers seem grossly exaggerated. Worldwide Skadden brought aboard ~200 this year, which was a big crop. The two years before that were ~100 put together. And over the last few years, the partners promoted counts have been: 2007 (29), 2008 (25), 2009 (8), 2010 (6), 2011 (8).
As for making partner in a BigLaw firm, folks I've talked to describe it as "a longshot" to "nearly impossible." A firm like Skadden will hire 500+ new associates a year (worldwide) and name maybe 4-6 partners, not all of whom started as associates. Unlikely the chance of going from associate to partner breaks 1%.
(Updated #s after asking someone who worked for Skadden.)