I think a 4 rating is pretty darn good at Demand because you never know how the CE's are rating you. I hover around 3.9 on grammar and 4.2 on research. The only thing I can think of that keeps my grammar from being "perfect" is that my sentence structure gets a little choppy when I'm writing for cheap. I wouldn't worry too much about improving your writing, unless it's costing you in rewrites. If they're not throwing the articles back at you, you're doing fine.
And if they really wanted to help us improve our writing, they'd let us know what they're marking us down for!
Once you're a little more established DS drops some of the limits on you. I can claim 20 titles, and claim more as soon as I turn them in for review. I've been there since last winter. I requested that my limit be bumped up after I worked there a few months. I don't hang out on their forums anymore (I've only got time to hang on this one) so I don't know what the current chatter is.
As tempting as it is, I don't know that I'd suggest titles anymore. Way back when they offered the same for your own titles as they did for craptastic ones. Five bucks is just way to cheap. $15 is bad enough, but most of us do it because it's reliable income and pretty easy work.
A tip for newbies: don't pick titles you need to do serious research for - it's just not worth your time and effort. If you need to, spend more time FINDING the good titles that are in your area of expertise and that you can write without brain strain. That's the only way you're going to make DS work for you. Use the keyword search, and try checking at various times of the day. Some people used to have great luck finding juicy titles in the middle of the night. I think this is either when they released the new stuff, or when titles would expire from other peoples ques and go back into the system.