Updated 14th July 2008
Lots of discussion about this on the MEDLIB-L list, and lots of detail of the changes on Laika Spoetnik's blog - Part III of her discussion is here with links to the previous two parts.
I wrote this, to go to the departments I liaise with:
1. The sidebar has gone from the summary results screen (to make way for “forthcoming features”. I don’t know what these are yet!). In some library circles this change is generating some controversy – if you don’t like it, let me know!
2. Advanced search. This allows you to search by field (author name, journal title, etc.), to set limits and to see your search history (limits and search history are available via the “old” routes too). Look for the “Advanced search” link next to the search box.
3. Changes to automatic term mapping (ATM). Previously, if you searched for a journal title or author name, but that title or name was the same as a MeSH term, you would get a subject search instead of what you wanted. To address this, a change has been made to the way ATM works. Previously, automatic term mapping would map your search terms to MeSH and to “text words” (words in titles and abstracts). It will now map to MeSH and “all fields”.
I think this might now introduce irrelevant references into your search, if your subject matches a journal or author name, but it does address the original issue. There is some controversy in library circles about this change, and so I would be interested to know if this has adversely affected your searches at all, or to have any comments. There are full details of the change at http://www.nlm.nih.gov/pubs/techbull/mj08/mj08_pubmed_atm_cite_sensor.html
New information about ATM: there is now a FAQ about it: http://www.nlm.nih.gov/services/pubmed_atm_change.html
4. Citation sensor: matches searches with citations if you use terms that can be interpreted as year, author name, journal title. Citation sensor results are displayed in a yellow area on the results display. This will not work if you use [au] and other tags in your search.
More details about citation sensor are at: http://www.nlm.nih.gov/pubs/techbull/mj08/mj08_pubmed_atm_cite_sensor.html
5. Title sort for results (does not ignore articles). Sorting results was possible before, but the title sort is new. You should know that it does not ignore initial articles, so a paper entitled “A new method…” will file under A, not N.
I have since discovered that the "old" spellchecker that used to say "Did you mean..." and offer a link to what it thought you meant has seemingly been replaced - I typed pharmacokinetics cucumin and it automatically told me there were zero results, but that it had searched pharmacokinetics curcumin instead, and showed me those results.