I need to thank Digital Inspiration (Amit Agarwal) for excellent articles and was blown away to learn this about GMAIL. I am not going to re-write all the things which is in the article, so I am going to summarize and point you directly to the article.
1) There is a limit on attachement(20 MB) and overall 6GB(although the ticker is always growing - may be will stop if MS fails to buy yahoo for $44.6 Billions).
2) You can send one email to only 100 people max through POP or IMAP clients, where as if you use browser you can send upto 500 people. I wonder if it is because most of the POP or IMAP clients used are Microsoft outlook!
3) You cannot send more than 25 people with in correct addresses.
Source - Digital Inspiration
