From chat:

(07:32:52 PM) subbu: i just checked the updated wiki page .. and  <hr> and colon-indents seem different in <poem> vs. indent-pre .. probably all wikitext elements that occur in SOL position but are strict about not having any leading ws.
(07:33:16 PM) cscott-free: how many of those are there?
(07:33:36 PM) subbu: i suppose <poem> just calls regular parse on embedded content and then wraps it in wrapper with the right css classes
(07:33:43 PM) subbu: tables are fine with leading space.
(07:34:05 PM) subbu: headings, lists, hr dont work if there is leading WS, iirc.
(07:34:41 PM) subbu: so, we were wrong .. <poem> is not indent-pre with classes then.
(07:35:10 PM) cscott-free: but should it be?  i don't like the idea of adding a bunch of new special cases to the parser.
(07:35:48 PM) subbu: cscott, it cannot be without breaking <poem> behavior OR breaking existing pages that have leading space before :*;#=- chars
(07:35:59 PM) cscott-free: if we can at least parameterize it, such that indent-pre has SOL markers turned off, and <poem> has SOL markers turned on, that would make me happier.
(07:36:14 PM) cscott-free: rather than poem just being gratuitously different in random ways.
(07:36:24 PM) subbu: ya, you are talking of refactoring code in the parser.
(07:36:30 PM) subbu: and reusing it.
(07:36:56 PM) cscott-free: subbu: i'm certainly talking about reusing more parser code, rather than implementing <poem> as a brand new crazy thing.
(07:37:25 PM) cscott-free: i'm tired of having weird corner cases everywhere.
(07:37:50 PM) cscott-free: there should be some principled way we can describe how indent-pre and <poem> are similar.
(07:37:57 PM) subbu: maybe spend some time, how <poem>/<lines> would fit in parsoid land and that can inform the discussion of how the core code can be restructured .. but refactoring/restructure that seems like a lot of work as well.
(07:38:33 PM) cscott-free: in a nutshell that's why i don't want <poem> merged too quickly.  i'd like to understand better how it all fits in.
(07:38:53 PM) cscott-free: and probably to do some dumpGrepping to figure out how much we can change indent-pre and/or <poem> without breaking too much stuff.