Is a separate tablespace for every table and index a good idea?

You will have to monitor freespace on all of the data files. The more data files you have, the greater the chance you have of losing or corrupting one. Oracle's entire architecture is built upon the multiple table and index per tablespace principle. I have never heard of anyone implementing one table or index per tablespace in Oracle.

Every time Oracle executes a system checkpoint, it writes to the file headers of every file. Checkpoints will be much slower.

Note: Thanks go to my good friend and Oracle guru, Chris Foot, for his help in preparing this answer.