Well, if you increase vitality to 19 or 20, your candle of life gets bigger.. . .
*wink* *wink*
nudge, nudge
(giggle)
Sorry, I just couldn't help myself, but it is true. I've not observed any other in-game effects.
Checking the Forge reveals the following:
Increasing strength will continue to increase carrying capacity, although by not nearly as large a margin as the increase from 17 to 18.
Increasing intelligence OR piety over 18 will help to make up the necessary 36 points (total, between piety and intelligence) necessary to get a +1 to academia skill points on each level up. However, if these two attributes already total 36, no additional benefit appears from raising either one higher.
Increasing vitality continues to increase carrying capacity and increases stamina points gained at level up. It will not increase hit points at level up. (But, again, you can brag about the size of your candle.)
Increasing the combined total for vitality and dexterity to 36 generally appears to give characters a +1 to physical skills on each level up. (Sometimes, this is a straight +1, such as mages, who go from 1d4+1 to 1d4+2; sometimes, it is a change in the die, such as fighters, who go from 1d5+3 to 1d6+3. Note: I didn't check this for every class; I've been performing all of these tests on fighters, but this peculiarity led me to look at some of the other classes.) Increases above that combined total of 36 appear to have no additional effect. (Technically, it looks like , if you raise the combined total to 42, you gain another +1, but I believe there is no way, in game, to raise any attribute over 20; you'd have to hack.)
Increasing speed above 18 will increase initiative.
Increasing personality . . . why would ANYONE care about increasing personality?? Other than meeting class requirements, it appears to have no effect whatsoever in the game, regardless of score.