YouTube support has been considerably enhanced in iPhone OS 3.
1, now, when you exit a clip (tap Done), instead of returning to the list of clips you were at, you’re presented two brand new tabs:
A, an “Info” tab with the list of the comments and other info (tags, category, date, description).
It also has a button “Rate, comment or flag”. As you may have guessed, this allows for rating and commenting (too bad the latter doesn’t work in landscape).
B, a “More Videos” tab with the videos of the same uploader; this also allows for subscribing to the given uploader, assuming you’re signed it. The subscription will appear immediately on your account.
2, I’ve already mentioned this in bullet B: you now can log into your YouTube account (under the “More” icon). After logging in, you gain access to not only the standard menu items (Most recent / Top rated), but also History, My Videos (a list of videos you’ve uploaded), Subscriptions and Playlists.
With these enhancements, the only (not very annoying) difference between it and Windows Mobile-based Kinoma Play (which I consider to be currently the best and most featureful, albeit, video playback-wise, not the most efficient player on mobile platforms) is iPhone’s YouTube not supporting changing the metadata (category, description etc.) of your own videos. This is, again, a minor disadvantage: Taking into account the worse video quality of Kinoma Play (QVGA resolution, as opposed to the higher HVGA on iPhone), I think I'll stick with the iPhone for the time being when it comes to YouTube playback now that, at last, I can browse both the comments of a video and the videos of the same uploader.
The biggest problem with iPhone’s implementation is that via 3G (UMTS / HSDPA) it seems to force the user to see the dumbed-down, low-quality (low resolution, mono – as opposed to the high-quality H.264 HVGA with stereo sound default delivered via Wi-Fi) video instead. There’s no way of disabling this. This is a definite restriction compared to the Symbian S60 / Windows Mobile implementations, where 3G users aren’t forced to put up with the low-quality stream. At least this is the case with T-Mobile, using a 3GB data plan, on a locked, non-jailbroken OS3 iPhone 3G. I assume this is the same with all the other carriers and even factory unlocked / jailbroken / yellowsn0w’ed phones.