If you're referring to the design of a complex infrastructure both hardware and software parameters come into play. Mainly bandwidth, security and Drive space. As companies or organizations grow the need for more bandwidth, increased security and more storage space are all factors that should be addressed.I'll try to answer these in three parts.
1) bandwidth - The need for more bandwidth is everchanging. Some companies pay-per-usage where as the some choose a dedicated or leased line. Do a search on T1, OC3 or dedicated leased lines.
2)Security - Where do I start? There are multiple levels of security a network administrator must consider. Server level, (ie sonicwall), email, user level and group are all factors to consider when creating an overall good security system or architecture. An admin may filter out adult content and games but allow the media dept access to facebook and twitter. He/she may also limit file size/type in email transfer.
3)Storage space - First why don't employees EVER delete old files and emails? This is a problem I run into occasionily. A network admin can't keep up with the constant changes an organization makes. This requires random housekeeping on our part which is not a fun task. An admin can set quota limits and restrict programs and email space but then I become a babysitter and we don't want that. Try to only allow groups certain access to certain places on your network and be sure to have a server side homefolder for each user/group then it becomes easier to manage. HOWEVER, cloud computing is here to stay. This will ensure storage space, programs and files become manageable. What I mean by that is storage space can increase when needed, programs are kept up-to-date without admin's wondering who is using what and files are backed up and archives are compressed. With more companies moving towards virtualization some of these tasks will be easily managed. Search VMware for more info.