That's kind of what I was thinking about for the sentinel too, djinn, but I felt that once I got more options up that it would fall more in line with the average power level. The sentinel DR is definitely something I need to reevaluate, but I'd like to leave it as it is with balances based on future updates.
Edit: As to comparing it with the gladiator, it is about where I want it. The gladiator bonus should be lower than the sentinel's DR. The gladiator's role isn't primary damage dealer, so he shouldn't be able to simply negate the primary defender's class feature. If I do end up reducing DR on the sentinel, it will still remain higher than the gladiator's bonus from weapon of choice.