My jewellery store got the longest (on average) reviews, maybe 200-300 characters, but they were generally overwhelmingly positive, or if they had a minor gripe, it would still be accompanied by a high rating (there's no pos / neut / neg, just a 1-5 star rating, 1-3 is considered bad, 4-5 is good). The gripes would usually be informative via more detail as well (eg stuff like "I thought it would be X, but it's Y"), which is IMHO a lot better than only having enough room to essentially say "not what I expected".
eBay does have a bit of a different culture, so I wouldn't expect exactly the same results or behaviours here, but people are mostly the same wherever you go so I don't think they'll be vastly different, either.
You'll get people using all 500 characters here and there, I'm sure, but I think there's more good than bad to this change, even in instances where someone is going off in a negative comment (one thing I will give eBay higher marks for, is that they will more often recognise and remove feedback that contains provable lies, Etsy usually doesn't).
