Hmm.. I used to be a skinny guy till last year so I would answer the question on basis of my personal experience 
I am a guy with  5'11''  ht and used to be 50 kg of weight ( well now I have managed to 65 kg some how and still working on it :) so now I don't face any prob. )

So what bitter side of being skinny guy I faced were.

1. People used to look at me and smile like an idiot as if they have seen something funny. 

2. I was awarded by lots of new names by peoples for example. Haddi Raja ( King of bones), Sukhdi ( skinny/dried), kankaal (skeleton), Patlu (skinny), chusa aam (sucked mango) and it really felt bad, actually it hurts

3. People were always ready with long list of advice for what to eat, how to eat, when to eat as if  I had survived on air and water till now.

4. Some people will show all their medical knowledge on you and will ask questions like are you suffering from TB( Tuberculosis ) or colon cancer ??

5. you cant wear what you want cause either it doesn't fits you or it doesn't suits you. 

6. No one takes you seriously as if you don't exist. for example "hey let me sit here you will adjust anywhere". 

7. No one will feel love at first sight for you, you have to impress them by your character. 

8. when ever you are in a group, you will be the only topic of fun for everyone.

9. If weather is not good people will comment on you not on weather, like "hey don't go alone or put some coins in your pockets,wind may blow you away", "You don't need to learn swimming, cause leaves float by default. "
 
And the worst part 

10.  I once being followed by a group of some cheap guys and they made so nasty comments regarding gay things ( let me clear that I am not), I had no option other then rushing towards a crowded place, ( First time I felt that how girls feel when eve teased).