The zooming-in and out is a big downside, as I often tweak in or out from what Safari gives me as a default when I double-tap.
The scrolling deceleration brakes to a halt sooner than native apps. Again, maybe I'm just being fickle, but if the physics don't match the native feel, then it just doesn't feel right.
Don't understand why they created their own form of copy/paste.
The zoomed-out small fonts are completely illegible. I can't even make out header or subheader text, or titles of articles.
Startup time, at least for me, was about 3 seconds, compared to immediately for Safari. I just have the 3G, and I'm sure it would be much faster on the 3Gs.
When going from vertical to horizontal view, the page does not resize. I have to reload the page for it to fit to the full width.
Despite the innovations that I was very excited for, it's the little things, the details, that will keep me from using this app. To me, good execution is in the details. Some of the things above, for me, are simply musts. An example would be a car that must feel good to sit in, have sharp steering control, easy-to-reach controls and a solid-feeling gearbox. Extra-cool features without this are non-starters. These are the things Safari gives me which Opera lacks.