#1 for me on all web-based services other than Gmail is the ads they're in your face all blinky and annoying.
#2: Advertising signatures..They got some cajones putting ads in the body of my email
#3: Lock-in most don't make it easy to forward mail of because they want to show you as many ads as possible
#4: Lock-in part II POP or IMAP access is almost always an upcharge some of the prices are ridiculous.. I remember back when Hotmail used to be free to download via Outlook or Outlook Express.
#5: Spam: An email address I don't even use gets all kinds of porn spam, so much that I cannot use the account. Their spam filters only catch about 50% of it. To be truthful I only got the @live.com address when it first came out for an easy to remember MSN messenger handle every time I turn on the client I get unread email notice pop ups that are all spam.
#6: When they tie in with an instant messenger like Yahoo or MSN messenger the instant messenger doesn't respect your computers email preferences. Say Thunderbird is your default client. Right clicking on a contact and selecting email automatically opens their web-based email all in the name of showing you more ads.
Is that enough complaints about webmail?
