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Simple, Google will only display 65 characters in the SERPs pages and you need to craft your titles for the maximum benefit which includes getting click throughs.

Also If you have 100 - 150 characters then each word is diluted in strength (to a degree) and if it needs to be that long then your page isn't as focused as it could be, you may be better off with 2 pages targeting different aspects of the keywords.

Keywords at the beginning of a title tag are currently thought to be useful - http://www.seomoz.org/article/search-ranking-factors#ranking-factors

However correlation data shows keywords in title may actually have a negative correlation to ranking - http://www.seomoz.org/blog/google-vs-bing-correlation-analysis-of-ranking-elements - (On Page Keyword Usage section)

You won't really be penalized for it, but it's certainly not best practise.

Hmm, I've not tested conclusively but I can't think how a comma would adversely effect you. However, anecdotally, I've seen lots of sites move away from commas and pipes in titles so I'd have to assume that something was up.

I don't think stop words are being treated the same as the used to be with Google trying to figure out intent more and more, plus I'm not a huge fan of special characters (not that a comma is that special) in the title so unless it looked ridiculous I'd consider not using it.

I see.

Well, I certainly used to put a few keywords in titles using commas assuming it acted as a separator, however (and again anecdotally) I now write (as does everybody I'm comparing myself to) titles in the format "Cheap Books and Free Books".

Stick 'poker' into Google and look at everybody's titles. A few years back that would have looked like 'Poker, Online Poker, Texas Holdem | PokerSite" where as now it's "Poker and Online Poker including Texas Holdem at PokerSite".

However, stick 'travel' into Google and you'll see everybody still using commas.

So, as I say, I can't think how it would adversely effect you but I think with Google trying to extract meaning from people's searches that, in the long term, more natural sentences over a list of keywords would serve you better.

Yeah, if it doesn't work you'll probably be faced with a blank page or you may get a 500 server error or maybe a 310. It should be obvious pretty fast