I started with Hive and worked backwards. It gives you a nice SQL interface and allows you to do M/R operations on CSV files easily. Once you get the hang of it, going back towards raw M/R or even something like Cascading/Scalding might be less of a shock.

If you know Cassandra or other NoSQL, you can try your hand at Hbase. To do anything with it beyond adding or removing data from a key, you'll need to write an application of some sort. Cataloging tweets is a decently simple exercise.

In my work, the only time I accessed the HDFS directly was doing a put/delete of a flat file CSV that I was going to load into Hive. I'm not saying there's not use cases for using HDFS, just that in the set ups I've used, I've never seen it.