(1) Click 'My Library' and do a 'select all' (Ctrl+A). The rightmost pane will show the number of items. This works for collections too.

(2) If you type chrome://zotero/content/tools/csledit.xul in your address bar you get a pane to preview styles for selected references. Also, the Styles repository at http://zotero.org/styles has handy style previews if you mouse over them.

I immediately caught a few errors that resulted from the switch from Endnote by viewing selected items.

What kind of errors are we talking about? Wouldn't they be visible in the right pane, where Zotero always shows the metadata? Also, doing a search for Endnote import on the forums will provide you with the experiences of lots of others who have switched. The errors might be fixable before import (EndNote's RIS export is notoriously buggy).

Well, this is obviously partly a matter of taste, but having switched from EndNote to Zotero myself, I found the right pane to be entirely sufficient for the purposes of spotting metadata errors and/or omissions — and way better as a generalized interface solution because it lets you edit fields in place (instead of having to doubleclick an entry to bring up the edit window, as in EndNote).

I really think that a built in preview in Zotero is useful, cos it gives you immediate and holistic feedback about any item you are focusing on, whether any info is missing, misplaced, etc. This is of course very helpful for those of us who switch from other reference managers with a large library.
One error I caught with the preview function yesterday following Mark's tip was that all book editors in book-chapter entries were automatically turned into Contributors during my switch from Endnote. I couldn't see editors in those entries, looked at the right panel, and realised that I should manually turn each Contributor into an editor. And after that things were fine in the preview.
Another problem was pdf files. I followed the standard suggestions during the switch. It seems now that all pdf files have been turned into links in the URL field to their locations in my hard disk. This was ok for journal articles cos I don't need the real URL in the final format. But the preview yesterday of articles from online journals, databases, websites, etc. gave me returns like "Retrieved from file://F:\Bibliography\...". On top of that, access dates were combined into the Date field.
These may be caused by Endnote export, but without an easy way to catch problems or see each item in its desired format is like doing anything without the ability to monitor your own actions and give yourself online feedback.
I have over 3900 entries, and over 2000 pdf files. Formidable task ahead if I have to manually check and correct errors without a built-in preview panel. I wonder if anyone has similar experiences and how they did it.
By the way, I'm just a middle-aged academic who's not technology savvy. It was like a life-long dream come true when I realised how Zotero could integrate bibiliography management with note-taking. I made the switch only about 10 days ago. I'm sure I can work out the glitches with your help.

Thanks adamsmith and mark for the help. Mark's initial tips actually gave me the answers I was looking for. And because of that (the preview function), I discovered other problems I have not been aware of after the switch from Endnote.

I do not object to the existing preview panel at all. I think it's excellent. It's just that I thought having a preview panel on screen all the time would be more convenient in terms of online checking each time a new item is added. Going to the bookmarks and bringing the preview panel up each time does the job, but a little tediously.

Now that I found the two problems I mentioned above, I've been trying to catch up with the reading for a whole day today. I still haven't found a solution. Please help and point me to a link that helps me solve 1/ the editor-contributor problem, and 2/ all pdf files in URL field. I'm using the beta version of Zotero, Firefox 3.5, and I exported from Endnote X2.

As I was reading, I found Mark's comments in August about the majority of new users of Zotero possibly being intimated by the complexities of switching from Endnote. I am one of these intimidated new users. I spent so much time searching for the right program because I was not happy with Endnote being so slow at incorporating the note-taking function. I bought MS OneNote and found that it was not what I was looking for. After I found Zotero, I then spent days reading up about it, and finally came up with the courage to make the switch. I'm now so happy about what Zotero can do, and so frustrated that I have to spend so much time searching and reading and still can't get easy solutions.