Ranking the top 50 domain registars based on their traffic does not seem to be a fair comparison of “top registars”.

I think the ranking system should be: price, support, features, user interface.
Price: This day an age domains shouldn’t be over $10.00/yr. Some companies still do this and it works for them. Network Solutions did up until a short while ago.
Support: Support is always highly regarded when ranking any type of service. Thus it’s a given on my list.
Features: As you say, URL forwarding is a nice feature to have. I also like having DNS being run from the registar on some sites. Features, features, features.
Last but certainly not least, User Interface. This relates back to your post on “How Polite is Your Website?”. I have come across some registars who have terrible user interfaces and make it hard to find what you are looking for. Or word it in a way that makes you over look it. From my standpoint if you do not have a good user interface I would use your service.