There's so much like that. I'm going to try to write a blog post highlighting all the gems.

My favorite is:

IMPLEMENT_SERIALIZE
(
READWRITE(prevout);
READWRITE(scriptSig);
READWRITE(nSequence);
)

It's a C++ macro that implements reading and writing member variables to/from disk/network. It uses C++ templates to figure out the sizes of everything, so it winds up packing items efficiently. We used something similar in the gamedev industry, so this was a delightful surprise.

Ohh, no, I take it back. This is my favorite:

//
// Compact size
// size < 253 -- 1 byte
// size <= USHRT_MAX -- 3 bytes (253 + 2 bytes)
// size <= UINT_MAX -- 5 bytes (254 + 4 bytes)
// size > UINT_MAX -- 9 bytes (255 + 8 bytes)
//
inline unsigned int GetSizeOfCompactSize(uint64 nSize)
{
if (nSize < UCHAR_MAX-2) return sizeof(unsigned char);
else if (nSize <= USHRT_MAX) return sizeof(unsigned char) + sizeof(unsigned short);
else if (nSize <= UINT_MAX) return sizeof(unsigned char) + sizeof(unsigned int);
else return sizeof(unsigned char) + sizeof(uint64);
}

When encoding an arbitrary value, if it's less than 253, it only uses 1 byte of space. When you read a byte off the network, if it's 253, you know there are two more bytes to read. 254 = 4 bytes, 255 = 8 bytes.

Again, it's a small thing -- very standard tactic. But there are dozens of tiny, effective decisions exactly like this all throughout the codebase.

There's a secure allocator for wiping your private key so that it doesn't hang out in memory, with hacks to work around MSVC8 problems. Which academics bother with MSVC8 hacks?

//
// Allocator that clears its contents before deletion
//
template<typename T>
struct secure_allocator : public std::allocator<T>
{
// MSVC8 default copy constructor is broken
typedef std::allocator<T> base;
typedef typename base::size_type size_type;
typedef typename base::difference_type difference_type;
typedef typename base::pointer pointer;
typedef typename base::const_pointer const_pointer;
typedef typename base::reference reference;
typedef typename base::const_reference const_reference;
typedef typename base::value_type value_type;
secure_allocator() throw() {}
secure_allocator(const secure_allocator& a) throw() : base(a) {}
~secure_allocator() throw() {}
template<typename _Other> struct rebind
{ typedef secure_allocator<_Other> other; };

void deallocate(T* p, std::size_t n)
{
if (p != NULL)
memset(p, 0, sizeof(T) * n);
allocator<T>::deallocate(p, n);
}
};

Another gem (note the comments):

case OP_EQUAL:
case OP_EQUALVERIFY:
//case OP_NOTEQUAL: // use OP_NUMNOTEQUAL
{
// (x1 x2 - bool)
if (stack.size() < 2)
return false;
valtype& vch1 = stacktop(-2);
valtype& vch2 = stacktop(-1);
bool fEqual = (vch1 == vch2);
// OP_NOTEQUAL is disabled because it would be too easy to say
// something like n != 1 and have some wiseguy pass in 1 with extra
// zero bytes after it (numerically, 0x01 == 0x0001 == 0x000001)
//if (opcode == OP_NOTEQUAL)
// fEqual = !fEqual;
stack.pop_back();
stack.pop_back();
stack.push_back(fEqual ? vchTrue : vchFalse);
if (opcode == OP_EQUALVERIFY)
{
if (fEqual)
stack.pop_back();
else
pc = pend;
}
}
break;

Bitcoin is worth N billion dollars right now, and this codebase really is a billion dollar codebase. Is this really the work of a single person?