i have a unlocked iphone 4 bought from apple. everything works as it should.
Maybe your phone needs to be reset? I am new to iphone, so forgive my basic answers hope you get all the stuff working.

It is Speakout, not your iPhone. For general browsing speakout data is great, but there are various restrictions in place. For instance, the is a cap for file sizes when downloading, which I believe to be between 1&2 mb. So, when you try to play a youtube movie, the app automatically downloads a certain portion of the movie before it starts streaming to avoid stalling. If that initial bit exceeds the download limit, the movie will fail. From what I can tell, you are limited to about 2 mins of standard definition video.

The new 3G speeds are great, but the upload speed is still terrible, so any app that needs to steadily send out data, like voice apps do, will probably fail. The Speakout proxy is also very limiting as well and could also be causing issues.

But really, at $10 a month I wouldn't be complaining if I were you