I saw this in Signs in the sand blog I read from time to time, MS XSL team is doing a survey on their blog, first I want to thank Oleg for the pointer, second I want to say that this is a great idea in my opinion to ask the developer community on what features they need/want.
My own experience with XSLT is quite limited, I have worked with it a few years back, and did some things with it, but I feel a bit un-qualified to answer the questions - especially the size questions, since I haven't dealt with large size XML/XSL combinations. However, and this is something that I think was agreed in all the posted comments, when programming with XSL (and it is programming) there is often a need for more functions, string functions, math functions etc... but again, I'm not that knowledgable about the subject so I will let smarter people answer.
The great thing about this idea is that it is well trageted, as a software engineer working on our product I have three clients usually:
1. The end users - regualr people who get to use the applications
2. Integrators - 3rd party companies who want to use our product as a platform (API)
3. Internal - other products within the major product scope who need to work with the internal components.
The easiest of the three is the internal customers, I can always know exactly what they need and why, and usualy can offer solutions for them that fit exactly, since they are consulted and working with me.
The hardest are of course the end-users because there are so many of them and it is very hard to get a solid response. Need I say more?
The integrators are a mixed bunch, but also a solid one - they are technical people using your product, and using it for their own needs, if you do your job well they will have no complaints. it is easier with black-box components like the XSL component when there is a well defined spec that everyone understand - but still sometimes input on features is required - and the perfect thing to do is ask, and ask the ones using it - just like in this case. Up to this day, I've enjoyed working with the XML team's product, Keep up the transparency and interest in the developer community, and thank you!