My char has reached Level 40 and is now stuck on that level. During days of playing and many many quests I have not advanced in level. I guess I am not going any higher now and cannot advance in any way!

What to do?

Start using skills not at 100?
Or use: http://www.skyrimnexus.com/downloads/file.php?id=1175
Otherwise do you have other mods installed?

See what happens if you try leveling up with a console command. However, before you do that, tell us you're not indeed trying to level up with lvl 100 skills.

No that's what happens when you play the game. Happened to me as well, although I didn't complain because I was leveling up far too fast anyhow. Sneak always levels up superfast for me in all TES games, because it's kinda my playstyle to play cautiously. Then I smithed for money to buy the skills I felt didn't improve much at all which I needed (armor). And then the bow of course. I realized too late what was happening and added 1h and block as secondary combat skills when things got tight (good idea too, since I was low on magic resistance), while lockpicking and speechcraft continued to climb at a steady pace. From level 40-56 (where I quit), there were hardly any progress felt at all. And I only started questing around level 30. I'm a hunter and explorer, not intending to grind skills to gain levels - yet it always keep happening to me in TES games.

Somehow I think FONV leveling mechanic is better suited to my gameplay style. I can explore as much as I want, but the real rewards in leveling doesn't come until I actually "play" the content (quests) the game has to offer, thereby prolonging the sensation of progress. Which is why I want a leveling speed slider - not everybody is looking to race up their skill, yet we want to do what the game has to offer.

Are you using skills that are not at 100?