What would I change?
Inventory issues
Ad networks currently are pretty limited in what they offer and how. For instance, there are networks that do an excellent job at offering placements in environments that are very granularly targeted but don't offer anything other than CPM-based buys. They're good at that one thing. Similarly, other networks offer CPA/CPL programs but represent iffy inventory and often can't or won't disclose their represented sites, much less allow you to suppress undesirables. This is problematic.
The ad networks that do rep blue-chip sites, in many cases, have to work with their sites to confirm inventory and special targeting options before responding to the agency RFP, which is time consuming, slow and frustrating for the sales side and the buy side.
Available lists
Additionally, none of the ad networks that I have been working with seem to offer access to dedicated HTML email lists. I presume that the publishers of these sites and lists feel that this inventory is too valuable to turn over to a network at low cost.
Demographic targeting
And although the potential is huge for these networks, they don't seem to have figured out how to offer demographic targeting of their inventory.
Customized recommendations
Also, the sales people at these ad networks have the opportunity to do more consultative selling (and upselling) but seem mostly reactive to RFPs and respond with pretty generic, cookie-cutter recommendations. Particularly with the CPA/CPL networks, these guys must have a wealth of experience around what works, what doesn't, and how to craft the most successful programs (not just central to their inventory but perhaps offering insight on how the advertiser/agency team can devise the best offer, landing page, messaging, et cetera).
Minimum buy requirements
My biggest pet peeve is that these networks have a high minimum buy requirement -- and are inflexible, at that -- so much so that smaller (or more cautious) advertisers, who would benefit greatly from access to the various contextual targeting and CPA/CPL opportunities for the purpose of learning online marketing's feasibility, can't play. In fact, one network insisted that if we wanted a guarantee of impressions for each ad size in the buy -- we were setting up a statistically readable test buy across creative executions and sizes -- they demanded their minimum buy of $25,000 net per each ad size. This ostensibly is there to cover the administrative effort of entering three "campaigns" into their ad server. Needless to say, they were dumped from our consideration set.
What makes the perfect ad network?
Just like the general merchants of the old West, the optimal ad network would have a little (or a lot) of everything that you would need in order to ride out the winter. Oops! I mean meet campaign goals: Some standalone, premium CPM-based site inventory and aggregated "smaller" sites are sold on a high-volume, low-cost CPM basis, as well as CPA/CPL/CPC offerings. Dedicated HTML email and a more consultative sales approach would all make me much happier.