A comparison of the _db_schema.inc.php files in 1.8 and 1.9 shows the following differences:

1.9 is missing:
from table T_blogs
blog_allowpingbacks
blog_pingb2evonet
blog_pingtechnorati
blog_pingweblogs
blog_pingblogdotgs

from table T_posts
post_flags

from table T_basedomains
INDEX dom_type (dom_type)

from table T_useragents
INDEX agnt_type ( agnt_type )

from table T_hitlog
INDEX hit_datetime

the upgrade function at line 476
this functionality appears to still exist in the new version, so it's been added elsewhere I assume

1.9 adds:
to table T_posts
post_notifications_status
post_notifications_ctsk_ID

to table T_hitlog
new INDEXes
INDEX hit_agnt_ID, hit_uri and hit_referer_dom_ID

new table T_item__prerendering, which is described as a prerendering cache table

to table T_plugins
plug_name and plug_shortdesc

1.9 alters:
table T_locales
loc_datefmt and loc_timefmt from varchar (10) to varchar (20)

table T_hitlog
hit_referer_type adds 'self' and 'admin' to the values in ENUM

I am inclined to say that there is nothing schema-related that should dissuade us from upgrading to 1.9.