Memory; it's Java. Java is great, but is a memory hog. Doesn't matter so much these days (well it does, but what are you going to do... There isn't much OSS competition for Hadoop). The documentation is horrible though. I'm not sure if this is a 'new' OSS trend, but now that i'm working a lot with Rails and gems I notice that on that side of OSS it's pretty normal to produce no or completely horrible documentation; use the force read the source and that kind of hilarious tripe. Apache projects which are Hadoop related (Hadoop, Pig, Hbase) all suffer from this; you are hard pressed to find anything remotely helpful and not incredibly outdated. At least for rails you can find tons of examples (no docs though) of how to achieve things; for Hadoop/Hbase everything is outdated, not functional and requiring you to jump into tons of code to get stuff done.

Again; there is not so much competition for tasks you would accomplish with Hadoop (and Hbase) on the scale it has been tested (by Yahoo/Stumbleupon and many others).