Reproduced on 4.3.0f2

The new WWW.bytesDownloaded feature completely freezes Unity until the download completes.

This feature was added per Case 459257 as a way of querying download progress as the download is happening.

---Steps to reproduce:---
I've put together a test scene called Test BytesDownloaded (under the folder of the same name). If you open that, look at the camera, uncheck the "Use Bytes Downloaded" checkbox on the Test Bytes Downloaded script, and run it, you'll see 10 frames of input, each printing the time of the frame and whether the download has finished. I used a 200 MB download as a test, and 10 frames of output, so it shouldn't finish in that time.

Now change the "Use Bytes Downloaded" checkbox to true. Hit run. Unity will completely lock up for as long as it takes the download to complete (~10 second on our network, probably much longer for you). When the download finally completes, you'll see 10 frames of output. All frames will show "done: True".
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P.S.: You can ignore the stuff under the Load Texture folder - that was from an earlier bug report.