Here's the place to start...

Check the tire pressures and level the deck on flat ground. The leveling tool in the mower height knob is a little iffy.

Pick up this leveling tool at the JD dealer... http://www.rungreen.com/John-Deere-Mower-Deck-Leveling-Gauge_p_1062.html

This tool sets deck level based on the blades not the deck shell.

{{gwi:345045}}

Maybe blades are worn out,as they do get shorter with age.

I would think that either your deck is dirty (grass clumps underside) or you will have to resolve this by changing the pitch front to rear.

Just a followup: As it turns out, all of the above was a problem. My tire pressures were uneven and one was really low. The deck was higher on one side, and was not slightly lower in the front vs the back. And the height adjustment was way off. JD has a height gauge that comes with the tractor, and I used that on the deck. I did not use the leveling tool that attaches to the blade, although I plan to get it. Finally, the bottom of the deck had a good 1/2 inch of dried up old grass. I scraped that out too. So now it cuts well, and I thank you all for the help.

got a pic of the grass blades after using a sharp blade?

I don't have a picture, but I just went out today to look at the cut grass ends. They look kinda ragged, which is disappointing (like Wheely Boy above...). The blades sure seemed sharp, and I had a local shop do them that does a lot of the commercial accounts. I am sure there are other factors. Maybe I don't have the deck properly aligned.

Are you mowing with the throttle set to max? Often people think it is better to mow at a mid throttle point, but the manufacturers recommend full throttle (WOT - wide open throttle).

You may need to use a lower gear once you up the engine speed.

From the Toro FAQ:

What are the benefits of running your riding mower on full throttle?

Cleaner cut on the grass
More power, less likely to bog down
Less stress on the engine
Engine runs cooler
Slow engine speeds can result in hot spots on the cylinder

In addition, many of these machines have hydrostatic transmissions that generate heat during operation. They have a fan that keeps them cool and the fan is driven directly by the engine. Running the engine less than full speed will frequently allow the transmission to overheat. The first sign is usually a lack of power on hills. Permanent transmission damage can result. By keeping the engine at full speed the operator will keep the fan at full speed. This will maximize the air flow for the transmission keeping it cooler and preventing premature failure.

Riding products should be at full throttle at all times.

Leveling the deck... see the second post in this thread.

To get the deck cutting level you need to level it based on the blades not on the deck housing.

If the deck is really driving as it should, it is back to leveling as lurker suggested. Side to side first, front to back next -recording the difference in height. The manual will tell you usually that the deck should be about 1/4 inch pitched lower in the front than the rear. Start there and keep records and pitch it to where it works best for you. Recheck side to side after adjusting the pitch.

Thanks to all for the help. I am mowing at full throttle. I will definitely check the leveling once more. JD makes a blade leveler tool, and I will get that since it is about $10. Some of it may be technique. I tried mowing perpendicular to my usual track, and it cut more evenly. Its also worse if the "grass" (weeds + grass for me at this time of year) is higher.