- High clock rate which means it
- ran very hot
- wasn't optimized for mobility
- not a notebook processor
- A "mobile" processor
- Optimized for battery life
- Specifically targeted for laptops
Nehalem vs. Sandy Bridge
Sandy Bridge uses a smaller nm technology for its circuitry over Nehalem, and on a per-clock basis there is some improvement in speed (< 20%). Other than that you won't find the large difference you found when going from Pentium 4 to Pentium M.
What will give you a big difference in battery life, power, and maybe even form factor is switching to Sandy Bridge and going with a SSD instead of a HDD. Battery life will be a lot longer, you'll use less power (with the same or better performance), and it may even be lighter.