Two words..."Armor Class".
The guy you described is awfully close to the character I've been playing for the last five years. At 20th level, I get 7 attacks per round (thanks to TWF, ITWF, and GTWF) and can drop a single creature in one round. However, I rarely do, for the following reasons:
1.) If the ACs are high, my iterative attacks tend to miss. (Especially the third and fourth ones).
2.) The need to move around generally prevents the use of a full attack action.
3.) My hps aren't exactly "tank-like", so I generally am forced to back off after that first round, or need to rely on Combat Expertise to improve my AC, thereby reducing my number of hits.
4.) With seven attacks per round, I generally fumble before finishing the round. We play that once you fumble, your round is over.
5.) I'm generally rendered useless by curses, negative levels, ability damage/drain, etc.
We also use the Imp. Crit and keen stack, and have found it to be a non-issue.
So my advice...crank the enemies' AC and throw in some clerics. Man, I hate those clerics and all their debilitating spells.
