Ran into some initial problems that I thought were due to upgrading directly from 4.7 to 5.1 (without installing 5.0 in between) - but according to the trusty folks at opensourcehost, it seems to have been a result of one or two contributed modules. Still, I think I'll do any future 4.7-5.1 conversions in two steps, to be on the safe side...
I've enabled all the "core" Drupal modules on this site, along with these "contributed" ones:
captcha - turing test for site visitors
cck - enables creation of new node types
devel - drupal development tool
event - creates event content type
eventrepeat - for repeating events
feedfield - creates feeds from nodes
form_store - used by captcha
htmlcorrector - fixes ill-formed teasers
image - creates image content type
masquerade - to view site as test user
nodewords - lets you add keywords and descriptions to the HTML headers
tinymce- an HTML editor
views - create pages and blocks based on taxonomy
webform - for online forms
weblinks - creates web link data type
weight - allows for ordering of nodes on a page/block