Well I am certainly not a excellent basemaker...I however think I'm good at C+Cing them
There are always common problem issues that I always see

Body Proportions: People have a hard time making the head proportional to the shoulders. And they have a hard time getting the arms the right length. Remember that from your wrist to your elbow is the size of your foot, this should reflect the same on the doll. Heads are a bit harder to explain, just look at your head in the mirror. How broad are your shoulders compared to the size of your head? Remember that women are curvy while men are straight as a board, and they do have somewhat of a breast...put we call them pecs, still, you shouldn't leave that area undefined.
Face Proportions: Oh gosh...I suck at faces...really...I really shouldn't be offering advice lmao. But from my own experience of getting CC on my faces, you need good size pupils and some sort of shading in the eyebrow and lashes. Not just flat pixel lines. As far as placement, I know from my how to draw manga book, the eyes should be level with the top of the ears. I'm going to post some links which you might find useful....cause I'm no good at helping haha.
Smooth Lines: When people trace photographs to make bases, they usually end up with jagged lines. This won't do. Look at your own limbs, how would they look if they were jagged and rough? Remember that you should continue in patterns, progressing into another pattern as you go, not single pixels then a three pixel line and then another single pixel. Also when tracing photographs, you don't see everything. Especially on the head. The hair covers some of the head which you need to draw on your  base. A lot of people stop the head at where a hair line was...so it leaves the head looking rather..cut in half. If you want people to use that base...give them a whole head, really....
Uncomfortable Positions: Some poses can look rather awkward.....and this isn't really something that is bad...it just looks weird. If you wouldn't be comfortable standing in that position..imagine how your doll is gonna feel!! lmao.

Thats really about all the CC I've ever really had to give except for the occasional progression of lines. But if you want me to CC your base for you, I'll gladly do it. I don't CC dolls very often..cause I feel like there are so many different levels of skill...its too hard. Bases, the levels are few and closer in range I think. But
 you can IM me on Aim: LorrTheLoser or on MSN: complex.enigma@hotmail.com or you could always email me too I guess kitty.popstar@gmail.com.

Some links I think you will find very helpful. Looking at how to draw manga is a good place to start learning about proportions and placing. These are also helpful in dolling itself as far as clothing.

http://www.polykarbon.com/tutorials/
http://www.howtodrawmanga.com/tutorial.html
http://drawsketch.about.com/od/animemanga/
http://otakuworld.com/links/howtodraw.html
http://hans.presto.tripod.com/scan/mangalinks.html
http://www.mangatutorials.com/
http://omu.kuiki.net/class.shtml
http://www.tutorialguide.net/arts_and_crafts/learn_how_to_draw/
http://www.animecubed.com/howtodraw/
http://elfwood.lysator.liu.se/farp/manga/manga.html
http://www.mangapunk.com/tutorials/
http://www.fanart-central.net/cat-3570.php
http://www.sailorenergy.net/HowToDraw/HOWTODrawSailorMoon.html
http://ver.rubberhouse.net/figure.html
http://ver.rubberhouse.net/tutorials.shtml
http://www.therisenrealm.com/art_drawing.html
http://www.wayuki.nl/basetut.html

I'll probably add to this with more that I find. I really recommend Wayuki's tutorial, the last one listed.