1) Email Deliverabilty
This is crucial to look into. Quite simply, many marketing automation platforms cannot get your email delivered - it goes to a spam filter. If the vendor is pushing price, price, price, be wary that they don't have a team dedicated to the relationships, procedures, and technology that is needed to get your emails delivered.

Some thoughts on evaluating deliverability:
http://eloqua.blogspot.com/2009/12/email-deliverability-health-check.html

2) Service Team
Marketing campaigns are time sensitive. If you are launching a campaign and need it to go out today, you don't have time to wait for slow support or emailed answers. A configuration question, best practice guidance, or technical challenge can delay you if it's not answered promptly. Make sure you look closely at how quickly the vendor *really* does answer the phone. Minutes is good, hours or days is bad. To evaluate this, just call them a few times at various times of day. Reaching a voicemail system is a bad sign. Make sure the person who answers the phone can answer a few basic questions.

3) Services and Implementation
Some vendors will show a flashy demo, tell you that you will be up and running in minutes, and hand you a piece of software without any guidance. This is a business process change, and without guidance on how to best engage with sales, score leads, manage data, collect insights on buyer behavior, or build nurturing programs that keep your audience engaged, you will likely make business process mistakes that have been made many times before. Look for a full success plan that guides you through to success. Make sure you get more than generic "training", you want to come out with *your* specific implementation set up for *your* business, AND have the skills to continue with it in your team.

4) Data Management
The success of *every* campaign is dependent on the data you are using, but because data management is not "sexy" it is often overlooked. Many vendors are quite weak in this area. Run through the following test in a demo:
http://digitalbodylanguage.blogspot.com/2010/02/evaluating-marketing-automati...

5) Performance
This is a critical area to look "under the hood" as many of the newer/cheaper platforms cannot handle big lists uploads or campaign sends without bogging down and making it impossible to do your work. Run through the following test in your demo: http://digitalbodylanguage.blogspot.com/2009/12/evaluating-marketing-automati...

6) CRM Integration
This is a broad area, but one to look under the hood on quite a bit. You will want to be up and running quickly, but still have the ability to configure the integration to fit your own unique business. All integrations are *not* the same, for many, many reasons specific to each business. Look for native integration, rather than using a third party vendor.

Some deeper thoughts on this topic:
http://digitalbodylanguage.blogspot.com/2009/10/evaluating-marketing-automati...

I hope that helps.
Steve