Is is ok to have a random thumnail of a sites homepage on your site including meta tag descriptions? I see this with some directories but in this case the site would not be submitted. It would be a site I show with positive a review or no review.
If it's a thumb, you should be okay. I remember reading a case about a site that did that, but with an artist's pictures. The thumbs were considered fair use, because the site was talking about pictures and the thumbs weren't deteriorating the market or causing harm to the artist's work. I do believe the site had links to the stuff so people could go to each picture and see/buy too. So, use the thumb, but make sure you're linking to the site. Of course, use a lawyer if you want official answers.
If it's for the purpose of review it's generally accepted that any quotation or screenshot can be used under the 'fair use' rules. I know there has been a legal case where someone tried to claim that web pages cached by search engines was a breach of copyright - the case got thrown out.
Thumbnails should be fine provided that they don't link to a full size screenshot of the site. It could be argued that the full sized screenshot could violate copyright and falls outside the "fair use" provision.