What I'm looking forward is to have a plugin automatically use the thumbnails in the posts in a way that makes sure that when users share the post on Facebook, the correct video thumbnail is attached. Another benefit I seek from trying Smart Youtube (PRO) is to have less steps in embedding a Youtube video in the correct way, which is important for the content authors. The ability to remove the annotations and the Youtube logo are also very welcome. Don't yet see what removing titles and ratings does so that might or might not be a benefit as well.
The only real problem I've bumped into so far is that while Smart Youtube otherwise simplifies embedding the videos, it requires the embedder to recognize the aspect ratios of videos. This is made more confusing by not calling them aspect ratios but "normal" and "high quality" modes. Video quality and aspect ratio are not the same - you can watch both 4:3 and 16:9 videos in any quality from 240p to 1080p when available.
What'd be optimal is that Smart Youtube automatically recognized the aspect ratio of the video, for example by querying the Youtube video page and seeing the embedding options available. Users would just copy-paste the link from Youtube, add the v, and be done with it. If instead the users need to distinguish between 4:3 and 16:9 then Smart Youtube makes things more complicated than they were before (users need to understand whether the options available require httpv or httpvh - less steps but more thinking).
Another way to improve Smart Youtube: the video sizes in Smart Youtube options are not what Youtube offers nowadays. For a 4:3 video the embed options are 420 × 315, 480 × 360, 640 × 480 and 960 × 720. For a 16:9 video the options are 560 × 315, 640 × 360, 853 × 480 and 1280 × 720. Although the user can input the desired size herself, it would seem appropriate for Smart Youtube to offer these sizes instead of what it currently has.