5800kbps is half of a DVDs bitrate
Not in practice, no. This might be why you're having problems believing that modern streaming is any good.
10Mbps is the maximum bitrate of DVD, not the average. Generally the average is around 3-4Mbps (if it wasn't, you wouldn't see any single layer single sided DVDs containing more than 80 minutes of content...)
But it doesn't stop there. That 3Mbps stream doesn't just include the video, it includes the audio. And on most DVDs, that's the core 5.1 stream in DTS (either 768kbps or 1.44Mbps) and/or DD (around 384kbps), plus the Spanish version (usually at least 384kbps), plus the director's commentary. So around 1-2Mbps of that 3-4Mbps (up to half!) is audio - and mostly redundant audio!
And the video... well, the video is compressed using MPEG 2 on a DVD. And MPEG 2 is difficult to optimize. Just look at the supposedly black background on the closing credits for example,
Now, with streaming, they've changed the codec to H.264 for the video, which addresses more causes of artifacts than MPEG 2. It obviously depends on the content, but bit for bit, the general consensus is that you can easily get equivalent or better quality out of H.264 over MPEG2 for half the bitrate.
And with streaming, they're only streaming one audio channel. They only need to stream one, as they already know which one you're going to listen to. So instead of including 1-2Mbps of audio in the stream, they only need to include 384kbps (less if you're listening in bi-speaker stereo.)
All of which means that they can go for a much lower streaming speed than you'd expect based upon extrapolating DVD video rates, and achieve much, much, higher quality.
I'm not going to argue that it's Blu-ray quality: when streaming, there's obviously the risk that your available bandwidth will drop and force the video to pause (Vudu) or drop to a lower quality (Amazon), but I would argue that, when I've watched videos without temporary bandwidth problems on, for example, Vudu (which doesn't implement dynamic bandwidth/streaming quality), the quality is good enough that 99% of people will never be able to tell the difference. It's a shame the Roku doesn't contain a hard disk, as it would be nice to tell the system what movie you want to watch, go off and make something to eat, come back, and watch it, knowing there's no risk of temporary bandwidth issues causing problems.