We've just upgraded to JIRA 6 and I noticed that now only the 5 most recent comments of an issue are shown and the old ones are grouped under the "x older comments" bar. I personally like this feature but apparently some of our users don't so I was curious if thi feature can be disabled or if there is a way to adjust the number of comments shown by default.
Please have a look at this and this page. I have not been able to test it (yet) but if you would add a "jira.comment.collapsing.minimum.hidden = x" replacing x with the value you wish, to the jira-config.properties file, this might provide you the solution you are looking for. Assuming you are using the download version of JIRA that is, for the OnDemand product you will no be able to make changes to or create this file.
I've searched some more and you can edit this in the Advanced Settings in your General Configuration Tab in the Administration Console. It's a property called jira.comment.collapsing.minimum.hidden
Some more information can be found here :
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