The other answers focus on the strengths/weaknesses of the Jedi themselves, and the overwhelming odds against them. I just want to point out that aside from being outmanned/outgunned, the Jedi were also hopelessly outmaneuvered.
Their predicament reminds me of the Battle of Agincourt, in which a superior force of Frenchmen were defeated by the tired and hungry Englishmen. You can read the Wikipedia article for a full description of the battle, but I believe there were a few reasons for the French defeat:
1 - They did not use their long-range assets.
"They had plenty of archers and crossbowmen but nobody wanted to let them fire [sic]. The reason for this was that the site was so narrow that there was only enough room for the men-at-arms." A different source says that the French did not even deploy 4,000 of the best crossbowmen "on the pretext they had no need of their help"
For whatever reason, the clones didn't get there until most of the Jedi were dead. Thus, the Jedi didn't have any long-range support, and the Geonosians could fly around all day and take pot shots at them.
2 - There were too many of them, and too close.
Rogers suggests that the French at the back of their deep formation would have been attempting to push forward and quite literally add their weight to the advance, without realising that they were hindering the ability of those at the front to manoeuvre and fight, actually pushing them into the English formation of lancepoints.
Jedi need a lot of room to fight. If you're standing anywhere in front of a Jedi, you could very easily get shot in the back by a deflected blaster bolt. Plus, with all their running and jumping, stabbing each other was a real concern. It seemed to me like the Jedi started holding their ground better when there were only about 20 of them, and they could stand side-by-side in a circle. Until then, there were just too many of them, too much chaos keeping them from moving tactically.
3 - The location was awful.
At Agincourt, the French marched first, through a hail of arrows and mud to the knees. On Geonosis, the Jedi jumped into the middle of an arena and surrounded themselves with the enemy. There have been cases where Jedi have done similar things and survived, but combined with the other two factors the terrain only compounded their failure.
So, in summary, the Jedi basically walked into a killzone, where there wasn't enough room to fight effectively, and where there were no clones to help them. There have been many battles like the one on Geonosis or the one at Agincourt, where a superior force essentially walks onto the enemy's spears. Usually it's due either to incompetence or overconfidence; I'd wager the Jedi had a little of both at the start of the Clone Wars. But, just like the clones, the benefit of organics is that they can adapt and evolve. In future battles, we see a smaller number of Jedi fighting alongside clones, and (mostly) avoiding being stupid. Which seems to work much better for them.
Also, I think there's a good comparison to be made between English longbowmen and Super Battle droids, but I don't know if it helps my argument.