Meh, can live without the sentinel, although combat specd sent would be nice for super speed and destroying tanks /healers(due to the 100% armor reduction ability they have)
Dedicated tank? Hmm, although they are nice, it's rare to find one who won't be a burden. For example I can be full dps spec and put a guard on my healer and still manage good dps. So bringing a dedicated tank is something you have decide for yourself if its very wise with the current group of players you're running with.