Most of this comes from the company I work for right now (as opposed to a general view). I can give you my general opinion separately if you'd like but it'll certainly be more longterm.
1. Antivirus / Antispam in Exchange. By and large we're sick of paying 100K/year each just to get protection for something which we'd rather be getting from Microsoft. We'd have no problem giving you guys 250K (an extra 50K) if it meant we got an integrated, supported, managed solution.
2. Greater connectivity and stability with other platform. The Connectors are good. More Microsoft ones would be better. More stability would be better. More error checking would be fantastic, especially on shared address books and calendars. All of our connected sites have to do manual cleans every year. On 50K entries that can be a big deal.
3. Caching / bandwidth throttling is becoming increasingly important. This might be more of an Outlook thing though...
I think those'd be our biggest ones, but that's mainly because the organization is (sadly) starting a push to Novell products.