We’ve put a lot of love into making this game an amazing experience. When it comes to systems, levels, and weapons, we’ve touched everything, because we made it all from scratch. We tried to keep what we felt was evocative of the original Half-life, but when recreating these things we have inherently re-designed them. The combat, the puzzles, the mechanics and pacing of every level have been re-designed. The weapon strengths, animations, balance aspects – all re-designed. The systems, aside from being completely coded from scratch, have all been re-designed to some degree. There is nothing we haven’t touched, honestly.
It's a good read and re-enforces the fact that Black Mesa: Source is very likely to be massive - Part One | Part Two