i agree, bookmaking tweets i like is a very big deal, i hope bokmarks end up being properly searchable or sortable in some way. twitter is a fantastic tool for me both for research (primarily) but also for promotion (hoping to crack it better)
bookmarking is a massive step forward for research/interest purposes.
not sure how share will work, but the concept is also great.
honestly i think twitter an incredibly useful tool, although it is much derided (and misunderstood), i'm very happy with this development!
I can't imagine needing to bookmark something three sentences long to, in their words, "read later."
On the contrary in my field (music journalism) many action points arise from tweets. If I spot somethng on my phone, it is so far down the timeline that I can have problems finding it when I am back at my computer.