I tested this on 3.6 Beta 1 cos that's what i'm running on my localhost, but it should work on 3.5.x
Admic CP -> Styles & Templates -> Style Manager -> Edit Templates -> New Posting Templates -> newthread
FIND:
Code:
<td><input type="text" class="bginput" name="subject" value="$subject" size="40" maxlength="85" tabindex="1" /></td>
REPLACE WITH:
Code:
<td><input type="text" class="bginput" name="subject" value="[Something descriptive here will yield more responses]" onfocus="if(this.value=='[Something descriptive here will yield more responses]')this.value='';" onblur="if(this.value=='')this.value='[Something descriptive here will yield more responses]';" size="50" maxlength="85" tabindex="1" /></td>