What are bottle necks in Load runner? Did you face any bottle necks in your previous projects? What is your approach in finding bottlenecks?

In simple words, Bottle neck is nothing but poor performance of the application. Example: you are trying to upload an image through your yahoo mail, and it is taking long time to upload, you (user) are getting frustrated with this delay of the application response time. This is clearly a bottle neck. But this bottle neck need not be yahoo applications problem, but depends on the internet speed your system is using.
There are normally different types of bottle necks, eg: memory bottle necks, network bottle necks, hard ware bottle necks, server bottle necks(Server bottle necks includes Application server, webserver & database server) and also application(code related, complex sql queries, nested queries etc..) bottle necks.
Memory bottle necks include Physical memory and or Virtual memory problems, memory leakage problems. Memory leakage can be found normally by Heap size. If the heap size is decreased gradually over time, then it is clear that there is some problem. If you are running a load test for long time, and you observe that the heap size is getting decreased then one of the problems could be memory leakage. Memory leakage means there might be a memory leak in the application. If garbage collection (GC) is not handled properly in the application (developers job), this memory leakage problems occurs.
Bottle necks mainly fall under any of the 4 areas. 1) CPU, 2) Memory, 3) I/O, 4) Software. This very important to know because then only you can start finding the bottle necks and solve them in a systematic order.