Don't use Salesforce. Don't even think about using Salesforce.
If you really must consider salesforce, read these questions and their top rated answers very carefully:
http://stackoverflow.com/questions/862294/what-are-salesforce-com-and-apex-like-as-an-application-development-platform
http://stackoverflow.com/questions/1664503/disadvantages-of-the-force-com-platform
If you're just looking to do something different, then consider making a single swap in the LAMP stack rather than throw out the entire platform. Writing Python or Ruby instead of PHP will feel substantially different and will open new opportunities. Or you could learn to use a NoSQL datastore, which would fundamentally change the way you model your data. Cloud computing is popular right now, you could look into building your next project on EC2 or App Engine.
I don't have any experience with SAP-ABAP, so I can't comment on it's quality. But philosophically, I would strongly suggest you stick with mainstream, open platforms that you know will still be around in 10 years. What will happen to your projects or your career when Salesforce goes out of business?