direct drive tt's?

It is very hard for someone who grew up in the cd era to know much about turntables, and what to look for and what to avoid. No area dealers have analog set-ups I can listen to, and I am the only person I know who even listens to records. So I thought I'd post here to gain a little knowledge hopefully.

I found a early 70's Miida direct drive turntable in the trash a few months ago. Model number 3115 I believe. The thing was in fine condition, very clean, and ran much quieter than the Thorens 140 belt-drive tt that I was using, so out went the Thorens and in went the Miida. But what I want to know is, what are the shortcomings of dd turntables? What kind of tweaks might be worth experimenting with to get the maximum out of the tt, or are such turntables not worth even fiddling with? What cartidges would be worth thinking about for such a tt, and for the quality of records I play (my collection is mostly from thrift stores and yard sales, but I do have a nitty gritty to clean my finds, and I also have some valuable records I certainly do NOT want to do harm to)? Is it possible or worthwhile to upgrade the tonearm on such a table?

Any advice and information would be welcomed. As I say, its hard to know much about vinyl playback and its subtleties when all you have ever seen are cd's and digital gear.

btw, I run the ol' retro-looking miida into BAT electronics and Snell loudspeakers.

thanks :)
pcanis