Dylan Boyd posted some more notes from the Email Insider Summit:

Myth: Subject line testing requires 1000’s of subscribers per test cell and several days to get stastically significant results.
Fact: Active "clickers" are VERY sensitive to subject lines. Email aficionados tend to check email often and provide insights quickly. Using very small samples of the right subscribers works well. You can test a small ACTIVE group and small INACTIVE subscriber list and see within 1-2 hours what each group would respond to.

Myth: All emails render the same properly across web based and desktop clients.
Fact: All emails clients treat HTML differently. Your
subscribers may have a VERY different experience with your email
campaigns, based on how and where they access their emails. Fonts all
work differently, links can change colors, extra buttons or links can
move or shift.

Myth: Delivery does not affect rendering
Fact: It does. When an email hits the bulk or junk folder in
many email clients, images and links will default to OFF. So if you are
not getting to the inbox, then you are not going to show up right at
all. In many bulk or junk folders it actually takes a few more clicks
to show the images, html and then activate the links.

Myth: Rendering does not affect deliverability.
Fact: The coding done in your email creative can send an email
right to the bulk/junk folder. When you write your code in a tablized
image map etc, it can reduce the chance of a good email ending up in a
bad place.

Myth: Opt-in Means Opt-in for life
Fact: Your Subscribers feel differently. At what point will your
subscribers work against your companies reputation? What is the cost to
mailing to EVERY subscriber? What is they are not performing, reading,
buying. Why keep them? Clean your lists.

As the consumer life grows on some lists (from a study) the length
of time on a list can actually grow the possibility of spam complaints.
At 31 months, 43% of a list were more apt to complain. Be introspective
and look at your files and take those non performers off your list.
Decide what is a good record. Don’t just have a list to have a list.
More important to have interest than risk a complaint or bad reputation.

Source: The Email Wars