Licensing v. Litigation
For those who think such patent trolls are merely suit-wearing under-bridge inhabitants found in kids' books, they are much, much worse. These trolls obtain the rights to patents and then profit through licensing and litigation rather than actually making stuff.
Personally I don't really see why it's worse that a non-producing company buys IP from a producer and licenses it, rather than a producer (like Microsoft) doing exactly the same thing. For me the issue with patent trolls is that, rather than simply license IP, instead they ambush small companies with claims that they're infringing and then effectively blackmail them with threats of litigation that a small company can't afford to defend.