Sometimes what's robbing your engine response isn't mechanical or fuel related. It could be a sensor problem. Most late-model cars regulate engine performance through a central computer. That computer uses sensors to look at driver inputs and engine conditions. It then directs various components based on the current conditions and what the driver is asking the car to do. If the sensors are bad, your car's computer is flying blind. It's kind of like blindfolding someone and then plopping them down in the middle of a marching band performance. You can help by shouting instructions, but because they won't be aware of all the conditions, they'll run amuck pretty quickly (Author's note: I'd pay to see something like this). A computer with bad sensors only has the driver's inputs to go on.
Two main sensors are usually the culprits for bad engine responsiveness: the mass air flow sensor and the engine speed sensor. The mass air flow sensor (MAF) measures and reports on the airflow into the engine so the computer can request the appropriate amount of fuel. If the MAF is bad, the engine won't be getting the correct amount of fuel, which will throw off the engine's combustion (we're talking about internal combustion engines, after all) and decrease engine response.
Though it sounds like it measures how fast you're going, the engine speed sensor actually measures how fast the engine's crankshaft is spinning. Just like you need a certain amount of air and calories to perform a given physical task, your car's engine needs a specific amount of air and fuel to operate at various engine speeds. If the computer doesn't know how fast the engine is working, it won't know how much air or fuel to send to it and the engine will lose responsiveness.
Diagnosing a sensor problem is usually fairly easy. A mechanic simply hooks up a diagnostic code reader to the car and is told which sensor needs fixing. You can get a code reader for home use too, though replacing a sensor at home can be a little tricky. Still, having your own code reader can make for some fun dinner party conversation, provided you get people's permission to diagnose their cars and don't just approach people and smirk that you have a MAF problem for them to solve.