Keep in mind, these answers are not from that of someone with absolute certainty, simply that I have played a long, long time.

>How much of a difference does Hit Roll actually make?

Generally, this is a stat that has a lesser impact on combat. Compare to damroll, for example, and you'll pretty much always take damroll over hitroll. Hitroll has been given some use in bypassing armor deflection skills...but that's rarely a character's main form of defense. It also does factor into the very basic "hit" vs "miss", but that threshold is usually only seen at very low levels.

>Is there a discussion thread someone can point me to?

This is probably most relevant to hitroll's more recent upgrade - though it still isn't a truly direly important stat (but it does also reference where hitroll is/isn't useful, by a knowledgeable implementor).

http://forums.carrionfields.com/dc/dcboard.php?az=show_mesg&forum=3&topic_id=1170&mesg_id=1170&listing_type=search

>Does it only affect the "automatic" attacks like 2nd and 3rd
>attack or does it play a roll in other things like some spec
>skills?

This may only be my perception, but I have always felt like some "active" skills did make use of hitroll reasonably significantly. It's quite possible it was my imagination, but things that required reasonable accuracy is where it seemed like it would make sense, and at times was helpful. It certainly can't hurt you to have better hitroll in those situations.

>Does a high hit roll make blows harder to parry or dodge?

Almost 100% certainty - no. That one's very much in the zone of the defender's skill at dodge/parry and the associated other factors. Shield block, however, is very much an armor deflection skill, which is what hitroll is specifically designed to do. So...maybe hitroll's the thing, when fighting a shield dedicate paladin?

>What is high enough? Is there a point of diminishing
>returns?

I'd say generally yes, it tends to have diminishing returns rather quickly. Admittedly, I haven't had to worry about hitroll at all for a few years, but if I recall correctly, most of my characters ran around with...maybe 20? And that only if melee was my main interest. Spellcasters/priests probably don't even have to worry about it at all.