The biggest difference isn't the billable hour, the work/life balance or even the amount of time spent on client development. The biggest difference is in the way we think. Law firm lawyers think like lawyers, exclusively. They issue spot and will list every conceivable outcome. They will agonize over the risks of a sloppily written indemnification clause and will argue for days over the psychology of selecting a venue. And they are very, very good at knowing what they don't know and seeking help when they reach that point.
On the other hand, in house lawyers think like an amalgam of lawyer and business person. We don't have time to analyze every little issue, just the big ones. We don't have the luxury of caring about the .01% chance that venue will come into play, because if we get there it's already done irreparable harm to our business. We make decisions based on half the information all the time, and often go with our gut when it would take too long to shepardize the latest case law on how arbitration clauses are being interpreted by the courts in various jurisdictions. This is what I call practicing law at the speed of business. If we don't get a 'good enough' answer now, they won't wait around to ask us for our perfect answer next time. So we speed through and take calculated risks (after fully disclosing and getting buy in on the risk profile with the company leaders). And we often suck at knowing what we don't know.
This is especially true for those of us who practice in small or solo legal teams that have to support every aspect of the business, often on a shoestring budget. And it makes our relationships with law firms so much more valuable. By being our sounding board on new issues when they arise, a law firm lawyer can position themselves as our partner in this journey and not just the legal expert in a particular area. By tactifully pointing out general areas that need to be considered - and not the laundry list of every single potential issue no matter how minimal - a firm lawyer can gain a reputation as being reasonable and practical. Which is great if you want me to send you business on those issues I need outside help on, and even better if you ever want me to recommend you for that in house opening that someone told me about.