40 ~50C is about right. i first had win7 installed running multiple 7z compression for 30min and it was running into 50's C with the whole case being pretty warm.
afterwards i installed pfsense, left it running for half a day and when touch it (i know this is very inaccurate way of measuring LOL) it feels about the same warmth. and it's winter!
anyway, i got a bit worried and checked d2500cc's manual, it specifically says being a fanless board, it requires some kind of case fan or the help of fan from power supplier, and that it should not be totally fanless. so, i'd say it's still better to stick a case fan into the case albeit a very slow one.