From what I've read, IBM and Microsoft were opposites, especially in code design. IBM wrote long, bloated code and paid by lines of code. Microsoft went for small, faster code. The documentary "Triumph of the Nerds" had an interview with Steve Ballmer about how IBM didn't seem to follow the mentality of paying a developer more if they were able to write a program in fewer lines, making it smaller and faster.
That, among other technical differences, was a big reason why OS/2 eventually become IBM only. I've never actually played with the Warp releases, but they sure look interesting, from what I've seen demoed.
IBM was (and still is) heavily enterprise-oriented - a legacy of their mainframe business. They didn't even market LAN Server (the server side of OS/2) very hard. Ordinary consumers and SMBs? They willingly - practically gleefully - steered those to Microsoft and LAN Manager (and eventually NT) before, during, and after the split - they wanted no part of such a plebian customer base. (I had, in fact, personally told IBM Mid-Atlantic's marketing mavens that if they didn't get their act together, Microsoft and Windows 95 would eat their lunch - in June of 1995.) OS/2 - as far back as 2.1 - was a squandered opportunity; OS/2 3.x - and even Warp (OS/2 4.x) were simply more obvious. (Look at eComStation - it is, in fact, OS/2 4.52 at the core. It's as least as viable as any Linux distribution, and supports more hardware than quite a few of them. However, even Ubuntu has a bigger word-of-mouth push then eCS does.)