I don't know, if you're worried about clutter, leave upvote as an arrow and everything else in a "Actions" dropdown.
It's really unfortunate they're not seperate - just now I "saved" a comment I KNOW I'll want to refer to later...went to my "saved" comments page and it's FULL (of everything I've ever upvoted obviously - I just now clued in that that's how you "save" a comment, although I already knew for articles).
Short answer: too much content, really horrible UI/UX for most browsers.
I use bookmarks for frequently visited sites, and a few select references. Using them for content management... really doesn't work particularly well.
Instead I actually download and save stuff I realliy want (and am actively looking for better tools to manage and organize that, though Zotero and Calibre are a start). I'd been using Readability somewhat, but it gets really inconvenient after a few hundred items (I've got 1000+ now).
At least with site-specific curation you've got stuff saved in some sort of context, though, frankly, that doesn't much work either.
It's a long-standing frustration of mine. Though it was actually kind of refreshing to read Vannevar Bush's Memex document a few days back which basically expressed the same concern from 1945.
I think separate buttons would be nice. The set of stories and comments I want to be able to refer back to is a relatively small fraction of the set I want to upvote.
Of course it also means another database table, or equivalent, in the back end.
It's not hard to save the data. What I don't see yet is a good interface for it. We're not going to pile on links for each new feature—that's obviously wrong for HN.
Edit: I have an idea. We could add a 'save' link, but only at the top of a story or comment's individual /item page. To save a story or comment, you'd click on its timestamp to go to its individual page, then click 'save'. This is exactly how comment flagging works now. It would add no complexity to story pages and very little to thread pages.
By that rationale, you can make Hacker News better by removing voting altogether. Get rid of those two arrows and the needless complexity of karma. Only you can save us from ourselves, dang.