I would suggest changing the term "Reply" to "Quote" to start with. From a usability point of view, I can see people who have never used the system before being confused between the two different terms "Reply" and "Multi-Quote".
It should either be: "Reply" and "Multi-Reply". Or "Quote" and "Multi-Quote".
Now that Multi-Quote is available, it seems a little inconsistent that one link is named "Reply" when in fact it is actually quoting the content of the post.
Reply requires nothing more than just clicking in the editor and typing.
Personally I have already made this change on my own site so it's no big deal to have an edited phrase but as it stands I feel there is some inconsistency in the naming and behaviour of these two functions.
I did come up with an idea recently involving the replay button though. I thought that the current one could be renamed to quote, and a new reply button(labeled reply) could be added, and when clicked, this would also embed posts replied to or quoted in the post you are replying to/quoting. Not the best idea, just something I thought I'd throw in.
I think the word Reply is still very appropriate IMHO because you are indeed replying to that post. The fact that it will also quote the post you are replying is extremely important in a conversation. Once you click the Reply link the first time, you immediately understand the power and great functionality of a forum and understand the difference between Reply and just typing in a new message using the Quick Reply.
I think the "quote" and "multi-quote" version sounds good. I don't like the + as it is not newbie friendly and to remain competitive we need to bring in a constant stream of newbies. I really hate the word "reply" as it currently exists. It leads to a lot of confusion on my forum. I find the following resource very helpful in fixing this: http://xenforo.com/community/resources/change-reply-to-quote.1539/