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1. By far the biggest issue, the lenses were not good enough. The Oculus Rift (dev version) comes with 3 sets of lenses for people with different prescriptions (one for people with perfect vision). Even if one of the lenses matched your prescription, there was a sweet spot for clarity. Any game that had you moving your head (read: all of them) jostled the Rift such that the lenses were no longer in the sweet spot. Things were blurry and it strained your eyes, especially after 30 or more minutes.
2. The straps. The Rift was heavy, and the straps didn't adjust well. We probably had special difficulties because we kept trading it around, but it was very hard to adjust the straps on your own and getting things adjusted perfectly took time. And if they weren't adjusted well, they compounded issues the with the lenses.
3. The resolution. The version we were using had a resoluiton of 1280x800, or 640x800 per eye, and the screen was so close to your face that pixels were easily visible. This was less of a problem on some of the demos, I'm guessing this was because they were made to be played at a low resolution. But when you played TF2 the resolution was a huge issue. Even if you could get the screen in focus (difficult at best), things were blurry and pixelated, and text was almost impossible to read. TF2 was probably the worst, but most non occulus-specific games suffered. The consumer version is supposed to be 960x1080 per eye, which is slightly more pixels than 720p TV, but the screen is so close to your face that I doubt it will bring the clarity of 720p TV.