I agree with both Charlie and DMV.
While both The Dave Clark Five and The Venturers were of debatable influence on Rock & Roll, they were some of the early groups in the genera, so eventually, the R&RHOF was bound to let them in no matter what, because that early on, almost every group was influential, just in varrying degrees.
Mellencamp - He has always run very close to line between R&R and Pop. Usually a bit more on the R&R side. Comes from the 80's and 90's when radio-friendly was the only way to succeed. We forgave Springsteen for some of the radio-friendly **** he produced and let him in, I think we can get over the "John Cougar-Mellencamp" days. I think Mellencamp does deserve a spot in the R&RHOF, but not in front of Leon.
Leonard Cohen (unless I'm thinking of someone else here) - He took blues and played it on a piano, he effectively single-handedly castorated the blues the same way "smooth jazz" mocked and insulted an art form. **** him, then cut his balls off just like he did to the blues.
Madonna? This is **** joke right? Did she bribe the voting committe? Madonna is living proof that scandal and sex sells, but this is **** ridiculous.