@hhl: the suggestion is nice, and it has been already thought over it couple of times, the issue is if we add another column next to replies, we will have very less space for Topic Titles. That is the main reason for not adding Topic Viewed option.
On the other hand it is not very useful according to me to have a topic viewed, u are only interested in a topic replies as that's what you read irrespective topic has been viewed 0 times or 10 times.
Regards,
Admin